Andradite

Silicate of calcium and iron, belonging to the garnet group.  It is named after a Portuguese mineralogist, J.B. D’Andrada.

Crystal system: Cubic

Appearance: Andradite occurs in crystals in the form of a rhombic dodecahedron, like all garnets, or in convex aggregates of crystals, whose color varies with slight variations in the stone’s composition, and which therefore have different names: the black variety is called melanite; the honey yellow variety topazolite; the green one, demantoid.  It can also be blackish brown or blackish red.  The crystals may be transparent, semi-opaque, opaque, and often have good luster.

Genesis: It is found in metamorphosed limestone and, rarely, in certain intrusive and extrusive igneous rocks.  The topazolite and demantoid varieties are found in serpentines, often together with asbestos.

Occurrence: Andradite is mainly found in alluvium in the Soviet Union and the United States, but also in Norway and Germany

         Amber

A fossil resin of trees that lived tens of millions of years ago.  Its chemical composition varies because it is a mixture of organic compounds, including succinic acid and succinic resins, originating from the polymerization of terpenes and resinous acids.  Amber is a noncrystalline.  The name amber has come down through French from the Arabic anbar.  The Romans called it succinum, as it was rightly believed to be from tree sap.  Its use as an ornamental material dates from Neolithic times.  It was brought to the foothills of the Alps from the Baltic coast.  The distances that had to be covered to obtain it, enormous for those days, give an idea of its importance.

Appearance: From transparent to translucent to semi-opaque, yellow to honey, brown or reddish brown, it sometimes has a dusty, friable reddish brown, light brown, or gray crust, due to alteration.  It is found in variously shaped nodules-swollen blobs, due to an accumulation of drops, icicles, run or large lumps, like those formed by the resin of present-day trees, although in far greater masses.  When present in alluvial sand or gravel, amber no longer has the opaque coating and is often rounded into pebbles or grains.

Occurrence: It is mainly found along the Baltic coasts of the Soviet Union and Poland, and in the Dominican Republic in the Antilles.  Other European sources include Czechoslovakia, Romania, and Sicily.  It is also found in the United States, Canada, and Chile; and in Burma.

         Amethyst

 The violet, purple to almost pink variety of quartz is called amethyst, and ancient name is derived from the Greek amethystos, meaning not drunken, as it was believed to protect those who wore it from drunkenness.  It is the most highly prized variety of quartz.

Appearance: The typical color is a rich, violet-purple, often distributed in patches or bands.  IT can also be quiet pale, but is generally the same basic color, without any overtones.  It is given both oval and drop mixed cuts, step cuts, and other types of cuts used for colored stones.  Specimens of good color but with too many inclusions are cut en cabochon.  Stones of 10 or so carats in weight are often found and even larger ones are not rare.  Amethyst normally has good luster and transparency.  Well-formed, characteristically colored groups of crystals are even used in their natural state as ornaments.

Distinctive features: The distribution of the color, in striking patches and bands, is characteristic.  When the stones contain inclusions, a series of discontinuous, wavy parallel lines, visible with a lens, indicates that they are certainly amethyst.  As with nearly all quartzes, the interference figure has a distinctive profile, which usually makes identification immediate.  Amethyst can also look vaguely similar to violet cordierite, which also has a strong, distinctive pleochroism.  The much rarer violet scapolite may look quiet similar, and its physical characteristics are almost the same as those of quartz.  Therefore an expert can only distinguish them.

Occurrence: The fines amethysts come (in great quantities) from Brazil and neighboring Uruguay, from the United States, Madagascar, and the Soviet Union, India, Australia, South Africa, and many other countries.

Simulated and synthetics: Amethyst was much imitated by colored glass in the past, when it was more costly.  Nowadays, despite the limited value of the natural stone, fair quantities of synthetic amethyst are produced, using the knowledge acquired in the production of synthetic quartz for technological purposes.  The cost of the synthetic product is not much less than that of the natural gemstone.

        Aquamarine

The name refers to the palish blue, light blue-green or even light green variety of beryl.  The green of aquamarine is a watery green without any trace of yellow and is due to iron, not chromium, as can be seen from examination with a gemological spectroscope.

Appearance: The most valuable color is a rich, sky blue; but because the stone is pleochroic, even the blue stones have a green or greenish blue tinge in one direction.  Quite large stones, ranging from several carats to more than ten or a few tens of carats, are relatively common.  Many are virtually free of inclusions.  The luster is vitreous and not exceptional.  The most common cut is the emerald type, although mixed oval or pear-shaped cuts are not infrequent.

Distinctive features: The color of this stone, combined with its particular type of pleochroism and vitreous luster, distinguishes it fairly easily from blue color, the second having a gray or violet tinge, much stronger luster and no pleochroism.

Occurrence: Most aquamarine comes from the pegmatites of Brazil, where crystals weighing several kilos have been found.  Other deposits are in the Soviet Union, Madagascar, the United States, and recently, Afghanistan.

Simulated and synthetics: Aquamarine is imitated by blue glass, which faithfully reproduces the color, if not the pleochroism, but it is most often imitated by blue synthetic spinel, of a slightly different color, with superior luster and no pleochroism.  Because of the general similarity, this is sometimes called synthetic aquamarine, although the latter, as such, is not produced.  Light green or yellow-green beryl can be turned blue by heating it to a certain temperature for a certain length of time.  This practice has been in use for several decades and is considered acceptable, as with zircon and sapphire.

           Aventurine

The name aventurine is applied to an ornamental material consisting mainly of quartz.  It is therefore described here under this heading; although it is actually a metamorphic rock, a quartzite, containing plate-like crystals of other minerals, usually green mica.  It is also, improperly, called Indian jade.  It should not be confused with aventurine feldspar, a red variety of albite.

Appearance: It is not normally transparent, but somewhat turbid.  Sometimes the green mica plates are obvious and greatly influence color, which may be an attractive, bright green.  More often, it is merely greenish off-white or grayish white.  The overall appearance can be quiet similar to that of some jadeite.  It is generally cut into curved pieces for necklaces or other jewels, or for use as pendants, but it is also much used for carving and figurines.  Because of its heterogeneous structure, it does not easily acquire a good polish.

Distinctive features: The granular appearance, the possible presence of distinct green fibers, and its particular translucence are the most distinctive characteristics.  Their density immediately distinguishes specimens similar to jadeite jade.  It is also much more brittle than jade.

Occurrence: Aventurine comes mainly from India, the Soviet Union, Brazil and Australia, but also from Germany.  It is fashioned in all these countries.

Simulated and synthetics: Aventurine feldspar is imitated by glass, usually in the form of a brown paste containing golden metallic fibers, hence somewhat dissimilar.  Some people claim that the name aventurine was originally given to a type of Murano glass containing metallic fibers and only subsequently applied to the ornamental mineral, which looks like it.  It is not manufactured synthetically.  Because of its granular structure, minute discontinuities and porosity, aventurine absorbs artificial colorants quiet easily, and consequently it is sometimes given a bright green color, with a view to greatly increasing its value.

                   Chalcedony

This is the name given to the microcrystalline varieties of quartz that form concretionary deposits.  They have been used since time immemorial both as gems, because of their color, hardness, and ability to take a good polish, and as precious materials for the production of ornaments or small sculptures.  The different combinations of colors and patterns have given rise to a specialized nomenclature that was once of great importance.  The name chalcedony probably comes from Calcedon or Calchedon, an ancient port on the Sea of Marmara, in Asia Minor.  Ornamental materials were mined in that area, and it was an active trading center for precious stones of various types and origins. 

Appearance: The typical color is blue whitish-gray, but for ornamental purposes, the types that have been variously colored by small quantities of other elements are usually used.  These colors can cover the entire mass, as with jasper, or just a few thin, successive layers, as with agate and onyx.  The most highly prized colors for the concretionary varieties, which are translucent to semi-opaque, are brownish yellow, red, black, green, black-and-white or gray-and-white and yellow, red, brownish red or black for jaspers, which are semi-opaque to opaque.  All varieties are cut into cabochons, engraved, or made into seal stones or rounded, polished, and pierced for necklaces and other items of jewelry.  Various forms of chalcedony were used extensively in the past for bases and handles of gold items and for stone inlay work.  Agate and onyx, with their consecutive layers of different colors, make excellent materials for cameos; the contrast between the different layers is used to heighten the relief.  Some variegated pieces are used for the carving of multi-colored figurines similar to those made from jadeite.  The most highly prized variety nowadays is chrysoprase, which is a bright green color.

Occurrence: Large amounts of chalcedony come from Uruguay and the bordering regions of Brazil, but it is found in many other countries.  In Germany, Idar-Oberstein is famous for agate, although the term agate apparently comes from Akhates, the Greek name for a river in Sicily where these stones were found several centuries BC.  Chrysoprase comes mainly from Germany, the Soviet Union, the United States, Canada, and Brazil.

Simulated and synthetic: In the past chalcedony was imitated by glass, moulded pieces even being used to simulate carved stones.  It is not produced synthetically.  Because chalcedony is porous, it has long been the practice to impregnate it with artificial dyestuffs, making it look like onyx, even where the original color is almost uniform.  This process is facilitated because the porosity of chalcedony often varies from one layer to another, so that one layer can absorb color well, whereas the adjoining one absorbs it little or not at all.  Agate has been artificially colored for so long and the procedure is so widespread that it is regarded as normal, not fraudulent, practice.  Chalcedony is also colored green to simulate chrysoprase.  This practice is considered fraudulent as the value of chrysoprase depends almost exclusively on color.  Equally fraudulent is intensification of the color of chrysoprase by the same means.

      Coral

Calcium carbonate in the form of calcite is the main constituent of calcareous corals; minor constituents are magnesium carbonate and proteinaceous organic substances, which act as binding agents.  A horny, proteinaceous substance whose composition varies according to the species is, on the coral used since antiquity as an ornamental material comes from the calcareous skeletons of colonies of marine organisms of the phylum Cnidaria, order Corgonacea, genus Corallium.  The most famous of these organisms is Corallium rubrum, which lives in the waters of the Mediterranean and, despite its name, provides not only red, but also orange, pink and white coral. 

Crystal System: Calcite, which is the main component of calcareous corals, crystallizes in the trigonal system.  The proteinaceous substances of the horny corals are noncrystalline.

Appearance: The skeletons of corals are vary in color: from bright to dark red, slightly orange-red, pink, and white; red to orange pink with areas of white, and from medium to deep pink, sometimes with alternate, concentric layers of lighter and darker color and a whitish portion.  Characteristic features are differences in translucency.  Sometimes coral has a different-colored marrow or medulla, or an axial canal.  Coral was believed to be a vegetable, a type of small, submarine tree.

Genesis: In all cases, coral consists of the branching skeletons of animals, which live in colonies planted on the seabed at depths varying from tens to hundreds of meters.  They are typical of warmish to very warm seas.

Occurrence: Banks of coral are found in the Mediterranean, along the coasts of China, Vietnam, and Japan; near the Philippines, along some of the many Pacific archipelagos; and along parts of the African coastline.  Coral colonies occupy large areas especially in the Pacific, but also near coast of South Africa, in the Red Sea, and to the east of Australia.  These latter colonies, however, consist of madrepore, which has little in common with the corals used as ornaments.

           Emerald

The name is of ancient origin.  The Latin smaragdus appears to have referred to the stone we call emerald, which is now considered as a distinct species.  It is basically the green variety of beryl, although not all gem-quality green beryls are called emeralds: yellow-green stones are called heliodors; soft blue green or even pale green specimens are called aquamarines.

Appearance: The typical color is beautiful, distinctive hue known, in fact, as emerald green and in sues to traces of chromium in the crystal structure.

But emeralds can be light or dark green, bright green or leaf green. The vitreous luster is not outstanding, and is strongest in medium-light stones with few inclusions. All emerald contains inclusions, although in the best quality stones, these are very faint and not visible to the naked eye. They show up under a 10x, 20x, or 40x lens. The most common shape for gems is the step or trap cut, which is also known as the emerald cut. They are occasionally given a mixed, oval cut, while antique stones are found with hexagonal, step cuts, cabochon cuts, or pear shapes with a hole in them, often used as pendants.

Distinctive features: The typical emerald color is virtually unmistakable.  Some very rare specimens of jadeite jade, which are less transparent and have different physical properties, only equal it.  To the initiated, the inclusions in emerald can be highly distinctive: a bubble of gas in a liquid, within spindle-shaped or, more rarely, truncated prismatic cavities; birefringent, circular plates of mica; multifaceted pyrite crystals or calcite rhombohedra.  However, a microscope is almost always needed to recognize them.  Although not the typical emerald color, some green tourmalines may look similar, but they can be distinguished either by their marked pleochroism, or by the fact that tourmalines which are given an emerald cut display alternating, longitudinal lines of lighter or darker color, when viewed through the table faucet. Olivine may also be a verdant green color vaguely similar to that of some atypical emerald; but the powerful birefringence of olivine is detectable with a simple lens, a double image of the opposite faucet ages being clearly visible in certain directions through the table facet.  In any case, the density of either tourmaline or olivine immediately distinguishes the stone from emerald.

Occurrence: The biggest and the most beautiful emerald come from the famous Chivor and Muzo mines of Columbia.  Much smaller quantities of emeralds, mostly of medium-light color, come from Brazil, and small, very intensely color stones, characterized by the numerous minute inclusions of molybdenite with a metallic appearance, are found in Transvaal.  In the last few decades, increasing quantities of emeralds have been found in a series of small deposits in East Africa-principally in Zimbabwe, Zambia, and Tanzania. These are quiet strong color sometimes with a bluish-green tinge; and they often contain mica plates and, sometimes, thin crystal needles.  The most famous of this emerald are the ones from Sandawana in Zimbabwe, which are value for their color.  Emeralds with similar characteristics also come from the mountains of India and Pakistan, as well as the Soviet Union, and formally Austria

Simulated and synthetics:The Romans are known to have imitated emerald with skillfully worked green glass.  Glass were also use in later centuries, extraneous particles sometimes being incorporated to simulated inclusions.  Doublets have also been used as imitations, with a lower portion of green glass and a top portion of garnet, or triplets, with a layer of colored cement sandwiched between two layers of colorless beryl, synthetic spinel, or quartz, Synthetic emeralds have likewise been widely produced over the last few decades.  Generally a good color, these are mainly distinguished from the natural variety by their inclusions and other features.  There are a lot of synthetic stones about, but their cost is quiet high, so that the market from them is saturated.

            Fluorite

Streak: White

Characteristics: Violet, blue, black, yellow to brown, green, pink, often zoned colors, seldom colorless.  Luster: vitreous; transparent to translucent. 

Cleavage: complete.  Fracture: conchoidal, splintery, brittle.  Flame test: dull red.  Often violet, blue, or green fluorescence under ultraviolet light. 

Aggregates: rough, sparlike, macrocrystalline to dense, rarely dendritic or crumbly; present in veins, pegmatites, and alpine crevices, as by-product of many rocks.

Found in: Harz mountains, eastern Bavaria, Germany, Austria, Switzerland, England, Spain, USSR, USA, and Namibia.

Similar to: Apatite, Barite, Chalcedony, Calcite, Halite and Apophyllite.

              Garnet

Silicate of magnesium, iron, and aluminum, belonging to the garnet family.  It is more correctly a group of minerals consisting of mixtures in variable proportions of the two end-members.  Mixtures in which magnesium clearly predominates over iron are called pyrope; those in which iron predominates are called almandine.  The name garnet, now applied to the entire family, was originally given to the garnets of the pyrope-almandine series, due to their resemblances to red pomegranate seeds (Latin name, malum granatum). 

Crystal system:Cubic

Appearance: Usually in isolated, granular crystals, often in the form of a perfect rhombic dodecahedron.  The color often reddish-brown, but can be a definite red, light red, violet red, or deep blackish red.  The crystals, which are often semi-opaque, can be transparent and limpid, with highly lustrous faces.  They have no cleavage.

Genesis: Pyrope is normally found in peridotitic and eclogitic rocks and also in diamond-bearing kimberlite.  Almandine is a characteristic mineral of metamorphic rocks.  Due to their resistance to weathering, pyrope and almandine are often found in alluvial, secondary deposits or are nacreous rocks.

Occurrence: This mineral is very wide spread.  Countries famous for garnets usable as gems include Czechoslovakia (Bohemia), South Africa, Madagascar, the United States, Mexico, Brazil, Australia, Burma, Sri Lanka, and India. 

          Iolite

Gem quality cordierite has the same name as the mineral itself, but was in the past also known as water sapphire, particularly but when the color was not really intense.  This is perhaps its most familiar name in the trade. 

Appearance: The color may vary from quiet a deep blue to violet blue, light blue, grayish blue; but it always has very strong pleochroism, being a much lighter gray or wan yellow in one direction.  For obvious, aesthetic reasons, gems are cut so that this color is visible from the side.  The most common type of cut is the rectangle, step type, not always with truncated corners. One also comes across cordierites with cabochon cut or minutely engraved, particularly in the case of less transparent specimens with numerous inclusions.  Most stone are a few carats in weight, not too small, therefore, but never very large.

Distinctive features: An essential characteristic of cordierite is its exceptional pleochroism, which may, however, resemble certain tanzanite.  Many cordierites have a decidedly cold, grayish coloration, whereas tanzanite is a warmer color, always with a hint of violet.  Testing the density, which is very different, would remove any uncertainty. 

Occurrence: Cordierite comes mainly from the gem gravels of Sri Lanka, but also from the United States and Namibia.

Simulated and synthetics: It is neither imitated nor produced synthetically.

          Jadeite Jade

The variety of jadeite pyroxene used as a gem or ornamental material consists of microcrystalline aggregates which, together with similar aggregates of tremolite-actinolite amphibole, come under the general name of jade.  For reasons of clarity, we shall therefore be using the term jadeite jade, rather than either jadeite or jade alone.

Appearance: Jadeite jade is a semi-opaque to highly translucent aggregate of juxtaposed, very firmly interconnected granular crystals.  Only in rare instances are there visibly elongated crystals, in fibrous radiating or parallel groups.  The general impression is not so much of individual crystals as of a mass with a fairly finely interwoven network of numerous, minute discontinuities.  These are the boundaries between one crystal and another or the cleavage surfaces inside some of the crystals.  The principle characteristic of jadeite jade is extreme tenacity and toughness.  On the surfaces of antique pieces in particular, one can see a similar network of small discontinuities, which keep the pieces from having a perfect polish.  On modern pieces polished with diamond powder, this network is much less visible.  Many different shades of green are possible, the most valuable being emerald green.  Jadeite jade of this color is quiet translucent and is called imperial jade.  Other shades of light and bright green are also very lively and attractive.  Dark green is less common.  The most frequent color is semi-transparent to nearly opaque white, or off-white to very pale hazel; others are red, yellow, light and often grayish blue, gray, pink, and pale lilac.  The brightest colors generally occur only in zones or in blurred, fringed streaks within the block.  Pebbles recovered from alluvium often have an outer layer of a brown color because of oxidation that fades away on the inside.  It is used as a gem, cut into cabochons or in engraved stones for settings, beads, other ornaments, especially when the background color or limited patches of color are in attractive shade.  Last but not least, jadeite jade, like nephrite jade, is made into exclusive types of decorated wear or small sculptures.  Because of its tenacity, which is almost equal to that of metals, generations of craftsmen in the Far East have used it to producing tours de force, such as chains with unjointed, individual links made from a single piece of stone, thin-walled vases generally with loose links on the handles and lids, cutting weapons, cups with engraved decorations, buckles, and other finely pierced objects.  Its other distinctive feature, the presence of patches and streaks of bright color inside and outside the uncut stones, has been skillfully manipulated into wonderful multi-colored pieces. 

Occurrence: Most jadeite jade used in Chinese art since the latter half of the eighteenth century and most of that used today comes from northern Burma.  It has also been found in Japan, the United States (California), and Guatemala, and only rarely in a few other places.  It is, therefore, much less common and more localized than nephrite jade.

Simulated and synthetics: Some whitish pale-green or green aventurine, also granular structure, is occasionally used as a substitute for jadeite jade and known as Indian jade, though it tacks the exceptional mechanical and chromatic properties of true jade.  Synthetic jadeite does not exist.  A problem that has arisen recently over jadeite is that of coloration.  Because of the minute discontinuities present in the mass, it is easily impregnates with artificial dyes, to pleasing effect.  In this way, for example, white jadeite can be made look like the much more highly prized imperial jade; and the other colors are also the often imitated in this way.  Once can usually distinguish cases in which the color, rather than being contained in individual crystals, is distributed in the minute fractures and cleavages of the mass but a microscope is needed for this.

        Labradorite

This is a sodium-rich plagioclase feldspar, which displays a particular type of iridescence on a dark ground.  The name labradorite is derived from its main source:  Labrador, in Canada.  The effect is probably due to the presence of very fine platelets of different compositions and minute inclusions of ilmenite, rutile and, perhaps, magnetite, which cause diffraction.

Appearance: The ground color is a dark smoke gray, but when light strikes it in a particular direction, it displays striking rainbow-colored reflections known as labradorescence.  It is cut into gems, or small not too convex, polished plaques for setting.  It is also used as an ornamental material for carving and engraving.  The background color is uninteresting and it is the strength of the labradorescence that gives the stone its value.  The particularly brightly colored variety found in Finland is sometimes known as spectrolite.

Distinctive features: It is highly distinctive at first sight; but there is an ornamental material, used in slabs and consisting of a rock containing large pieces of potassic feldspar, which looks similar to labradorite.  This material, which to called larvikite after the place where it is found in Norway, is used for building purposes only.  If necessary, the two could be distinguished by their densities.

Occurrence: The labradorite used in gems comes mainly from Canada and Finland.

Simulated and synthetics: It does not appear to have been imitated or produced synthetically.

      Lapis Lazuli

The name of the gem is derived through the medieval Latin lapis lazulus, from the Arabic word lazward, from which the word azure comes; but according to the descriptions of Pliny the Elder, the ancient Romans called it sapphires.  The name sapphire was subsequently applied to the blue variety of corundum.  Lapis lazuli is a “rock,” because it consists of an association of minerals.

Appearance: It has a uniform, massive or sometimes granular appearance, with fairly distinct crystals.  It is semi-opaque or opaque, with a surface that can take a good polish.  It is a strong but lively blue, sometimes with a hint of violet.  It often contains grayish or off-white patches or veins, consisting of distinct, interwoven crystals which are minutely fringed at the edge of the patches, interpenetrated by and interwoven with the minute crystals of blue.  The presence of white patches reduces the gem’s value.  The most highly prized varieties are those uniformly colored, preferably without a violet tinge.  It often contains minute, scattered crystals of pyrite, which do not detract from its value.  It is made into spherical or curved beads and even faceted, polyhedral ones, in which the flat facets can take a very good polish.  It is also fashioned into carved gems, boxes, mosaics, small ornaments, vases and figurines, the largest of which may be tens of centimeters in size.  The Egyptians used for their cylindrical seals.

Distinctive features: The particular, very attractive color and speaking with minute crystals of pyrite gave lapis lazuli an unmistakable appearance. 

Occurrence: The best quality lapis lazuli comes from Afghanistan, where it has been mined since remote antiquity.  The ancient Egyptians probably obtained their supplies from there.  It is also found in Chile, but usually with numerous light patches and veins.  Much smaller quantities come from the Soviet Union, Burma, Pakistan, Angola, the United States, and Canada.

Simulated and synthetics: It was and is much imitated, by glass, sometimes containing minute specks of metal to simulate pyrite, by stained chalcedony, and by deep blue sintered, aggregate of minute grains of synthetic spinel.  A product has recently appeared on the market, which is extremely homogeneous, very deep, blue with a violet tinge and scattered with minute fragments of pyrite.  This is called synthetic lapis lazuli, although it does not correspond exactly with the natural stone in chemical composition.  The white patches in low quality lapis lazuli are sometimes colored blue and this practice is not always easy to detect. 

        Moonstone

The Sodium-rich end-member of the plagioclase feldspar group, called albite, from the Latin albus, because of its whitish color, may look similar in appearance to adularia moonstone if cut en cabochon. 

Appearance: It is typically misty, semitransparent or semi-opaque with a pale, shimmering reflection, less well defined than in chatoyant stones.  It may be milky white in color, or dull yellow, yellow-gray or greenish gray.  It is almost always cut en cabochon.  Curved pieces are also cut as necklace beads or pendants.

Distinctive features: The adularescence is quite distinctive.  This is also found in adularia moonstone, but the two are distinguishable by their density, which is higher in albite moonstone.  The refractive indices, which are always hard to establish in curved stones, are slightly higher than those of orthoclase, with slightly stronger birefringence as well.  The yellowish or light brown coloration of some specimens also distinguishes it from adularia moonstone.

Occurrence: It comes mainly from Canada and Kenya but occurs also in India and Sri Lanka.  In the latter two countries, however, it is confused with the similar variety of orthoclase.

Simulated and synthetics: It is imitated by milky synthetic spinel, which, however, lacks the mobile reflection.  It is not manufactured synthetically.

         Opal

Noncrystalline hydrous silicon dioxide.  Opal has the same chemical composition as quartz, but contains from 1 to 2 percent water, and is not crystallized.  The name opal is apparently derived, through the Greek opallios, from the Sanskrit upala, meaning “precious stone.”

Crystal system Noncrystalline

Appearance: It occurs as narrow veins of up to 10 centimeters or more, or as nodules, inside cavities or cracks in silica-rich rocks.  IT may also be found pseudomorphous after other minerals.  It may have a whitish to light gray, pale green, sky-blue, smoke gray, black, yellowish to orange or reddish background color.  It can be semi-opaque, with a vaguely porcelain-like appearance that is so characteristic, it is described as opalescent.  Opals can even be fully or largely transparent; such stones are usually orange-yellow to red color.  The most highly prized varieties display internal iridescence due to light diffraction by the network of tiny spheres of which they are composed.  These types are collectively known as noble opal or precious opal.  The range of colors apparently depends on the size of the spheres or rather, the distance between the rows.  In gem quality precious opal, three sets of distinctions are made.  The first, according to the ground color of the material, distinguishes light or white opal from dark or black opal.  The second, applied to each of these two varieties, is based on the range of colors in the iridescent patches; and the third is the based on the side, shape and distribution of the patches.  The transparent or semi-transparent, noniridescent variety is also used as a gem if it is attractive colored.  Because of its orange-yellow to reddish orange color, it is known as fie opal.

Genesis: Opal is normally found in association with effusive magnetic rocks, such as rhyolites, andesites, and trachytes, having been deposited in cracks and cavities by awueous fluids at low temperature.  In Australia, it is found both in connection with trachytes and basalts, and in siliceous sandstone where hydrous silica has been precipitated, perhaps through alteration of feldspars by percolating waters in an environment subjected to very long periods of stable conditions.

Occurrence: One area of eastern Czechoslovakia formerly belonging to Hungary has been mining opal since Roman times and was the only source of noble opal for Europeans until the nineteenth century.  Nowadays, most opal comes from Australia, where the finest quality opals are found.  Other sources are Mexico and to a lesser extent, Guatemala and Honduras.  Low-value or subgem quality varieties of opal are found in many other places, especially the United States and Iceland.

          Quartz

Silicon dioxide.  The name may be derived from querklufterz an old German word apparently referring to whitish, vein quartz.  It is one of the most widely distributed minerals in the earth’s crust, sometimes found as elegant crystals whose luster, hardness, and watery transparency or, conversely, pleasing colors have long been a source of fascination, causing it to be widely employed as a gem or ornamental material.

Crystal system Trigonal

Appearance: The most typical form consists of hexagonal, prismatic crystals with pyramidal or bipyramidal terminations, which are transparent, colorless, lustrous, and have no cleavage; but quartz may have a smoky appearance or even be black, yellow, brownish yellow, violet, or pink.  Massive, white, milky vein quartz is very common.  A microcrystalline variety of quartz, found as compact, massive concretions, is called chalcedony; this has separate colors, sometimes with distinct color banding, in which case it is known as agate, sard, cornelian, plasma, etc., depending on the color, a whole series of names having been evolved during its long history as a decorative material. 

Genesis: The largest crystals originate from the fluids associated with intrusive magmatic phenomena and is found in pegmatic, pneumatolytic and hydrothermal deposits.  Quartz also occurs in sedimentary and metamorphic environments, but as very small crystals.  The microcrystalline varieties are often associated with hydrothermal processes, even under the sea. 

Occurrence: Quartz is extremely widespread, the most famous localities for magnificent, large crystals being the French Alps, the St. Gotthard massif in the Swiss Alps, the United States, Brazil, and Madagascar.

               Ruby

The most valuable variety of corundum is ruby.  The name comes from Latin rubrum, “red.”  Like other red stones, it has also been called carbunculus, or carbuncle, meaning a small coal or ember. 

Appearance: The color varies from fiery vermilion to violet red, but because rubies are pleochroic, different colors are also found in the same stone; bright or sometimes brick red in one direction, tending to carmine in the other.  The color is also accompanied by marked fluorescence, which is stimulated by ordinary, artificial light and above all, by the ultraviolet rays of direct sunlight.  Thus rubies turn brighter red under such light and the purplish ones look “redder.”  If he color is too pale, they are no longer called rubies, but pink sapphires; if it is more violet than red, they are known as violet sapphires.  But it is hard to establish precise limits, as all the intermediate shades are possible.  The brightest red and thus the most valuable rubies (usually from Burma) often have areas full of inclusions in the form of minute rutile needles, which interfere with the light, producing a distinctive silky sheen known, in fact, as silk.  When the silk is not heavy, the stones are clearer, more attractive, and even more valuable.  Rubies of this type are not usually more than a few carats in weight.  The rare exceptions generally contain copious inclusions.  Violet red, sometimes quite dark, rubies come principally from Thailand.  The type most often found on the market nowadays, they can be several carats in weight.  They are normally clearer, without patches of silk.  While good-sized clear stones are found, specimens with many inclusions are commonly sold as well.  Rubies are usually given a mixed cut, which is generally oval, but can be round or, more rarely, other shapes.  In the past, they were given a cabochon cut, like all stones outstanding for their color.  Today, however, this cut is reserved for less transparent stones with numerous inclusions. 

Distinctive features: Rubies can often be distinguished by their immediately visible characteristics: a fairly obvious pleochroism, a distinct brightening of color in strong light, the silk effect, and a considerable luster, it is not pleochroic, turns much less bright in strong light, and never displays the silk effect.  Red garnet is not pleochroic and the color does not brighten in strong light; it has a similar luster, but when given faceted cut often displays dark, blackish areas within the stone.  Red tourmaline is usually a completely different shade, but can be very similar, with a pleochroism comparable to that of ruby.  It does not, however, brighten in strong light, and this can be sufficient indication to warrant testing its physical properties, which are quite different.

Occurrence: The rubies with the finest color come from the Mogok region in Burma.  These are most truly vermilion, though they still have a touch of carmine.  Thailand, however, is today main source of rubies.  Thai rubies are usually slightly less attractive, a bit darker with a violet tinge, but they often have fewer inclusions.  Rubies are also found in Sri Lanka, but in very small quantities.  Often pale, almost pink, they can be attractive, with an appearance that is both brilliant and lively.  Small quantities of very fine rubies also come from the area of Cambodia on the border with Thailand, while rather opaque specimens, mainly of inferior quality, are found in India and Pakistan.  Tanzania and neighboring countries have also been mining rubies for a few years.  Some of the rubies found in these countries are almost as finely colored as those from Burma, with inclusions similar to rubies from Thailand, while others are semi-opaque and of very limited value.

Simulated and synthetics: Ruby has very occasionally been imitated by glass, which has rather different, less lively color and an inferior luster.  Doublets, have sometimes imitated it with the top part consisting of garnet, to provide luster, hardness, and natural-looking inclusions and the bottom part of red glass, fused rather than cemented to the garnet layer.  But such imitations are uncommon.  Synthetic ruby has been produced from the beginning of the twentieth century and was the first synthetic gemstone to be manufactured on an industrial scale.  To make these synthetic stones harder to distinguish from some natural rubies with numerous inclusions, they have sometimes been produced in the Orient.  The top part of these doublets consists of poorly colored natural corundum with obvious, typical conclusions; and the lower part is synthetic ruby, held to the corundum by transparent cement.  The effect is highly deceptive:  the reassuring presence of natural inclusions and characteristic luster combined with a color which is not perfect, but is normal for the majority of rubies, can be much more convincing than a synthetic ruby.

           Sapphire

This is the blue variety of corundum.  The name is probably derived, through the Latin sapphires and Greek sapheiros, from a Sanskrit word.  As with other gem names, however, the Latin sapphires did not originally denote the gem it is associated with today.  Judging by the description of Pliny the Elder, it almost certainly referred to what is now known as lapis lazuli, rather than corundum.

Appearance: Sapphires can be a very dark blue, to the point of seeming dense and blackish from a distance, sometimes accompanied by a blue to dull green pleochroism, which is only visible from the side in cut stones.  They may also be a strong, but not bright blue, easily recognizable from a distance, this being the ideal color.  Other possibilities are light, usually bright, blue, with the color unevenly distributed; palish blue or, finally, blue with a violet tinge, at least in bright light.  Sapphire always has good luster.  Some sapphires display clearly defined streaks or paler color, in contrast to a dark ground.  Others have areas with a slightly silky sheen, which are not clearly delineated.  Still other, uncommon varieties assume a distinct, milky appearance in strong light, with marked increase in color intensity.  Inclusions are less obvious in very dark stones, due to their general lack of transparency, whereas medium to large, pale stones often show distinct “veils” or “feathers” caused by very fine inclusions and foreign dark, sub-metallic, and opaque, and, very occasionally, bright red.  Sapphires are usually given oval or less frequently, round, mixed cuts, rectangle or square, step cuts, with or without trimmed corners, are also possible.  The cabochon cut is used as well, although less frequently than in the past.  Nowadays it is generally reserved for stones full of inclusions or those in which the color is concentrated in a few streaks on a light ground.  In the latter case, the cabochon cut gives the color a more uniform appearance.  Stones weighing several carats or even 10 to 20 carats in the case of light-colored specimens are not uncommon.

Distinctive features: Like other types of corundum, sapphires have a striking luster.  The color is also quiet distinctive, whether or not clear blue-green pleochroism is visible.  The overall appearance is very important.  A slightly patchy, blue color with imperceptible pleochroism and strong transparency showing veilike inclusions and a slight silk effect, still with excellent luster, denotes a sapphire from Sri Lanka.  Of the other blue stones, tanzanite always shows a hint of violet, fairly obvious pleochroism, and less luster tan sapphire.  Strongly colored specimens of indicolite tourmaline are often an attractive greenish blue, with a pleochroism ranging from blue to green, but the green is very different from that of sapphire which, when it is present, is always dull or yellowish.

Occurrence: The best sapphires were discovered in a small deposit in Kashmir in 1880, in a remote mountain area, which has now probably been exhausted.  Very fine sapphires are also found in Burma, but in limited quantities.  Appreciable quantities of light-and bright-blue sapphire are found in alluvial deposits on the island of Sri Lanka.  These are always attractively colored, the richest versions being very similar to the Burmese sapphires and equally valuable.  The sapphires of Sri Lanka are also famous for the variety of inclusions they display:  long, thin rutile needles, like very fine silk; soft, liquid inclusions arranged in the form of veils, lace, and feathers; striking inclusions with a moving bubble, like a spirit level; zircon crystals with small stress cracks radiating from them, and various other types of transparent crystals.  Sapphires are also mined in Thailand and neighboring Cambodia.  These are generally pleasing to the eye, though often rather deeply colored.  But most sapphires come from Australia, which has numerous deposits of deeply colored stones, sometimes too dark, in most cases with blue-green pleochroism.  These are the least valuable, but most widely available on the market.  Less important sources are the United States (Montana), Tanzania, and Malawi.

Simulated and synthetics: Sapphire has been imitated by dark to cobalt blue grass, but particularly by doublets with a top part consisting of red almandine garnet, which is very hard and lustrous, with natural inclusions, and a bottom part of dark-to-cobalt blue grass, welded together, not glued.  IT has also been imitated in the past by synthetic blue spinel, which is brightly colored but emits strange red gleams in bright light.  Synthetic sapphire has likewise been produced for many years now, mainly by the Verneuil flame fusion method.  Since the end of the 1970s, greater knowledge of the nature and causes of color in gemstones has enabled the modification of this feature by various procedures.  The most recent and important techniques relate to the blue coloration of sapphire.  One method is to subject very pale blue, almost colorless stones with numerous silk-like rutile inclusions to prolonged heating at temperatures in the region of 1500-1600 degrees Celsius in a reducing environment.  This “reactivates” the titanium in the rutile, which reacts with the traces of iron in the sapphire.  In this way, the silk is absorbed, while the trivalent titanium and iron thus formed, which are responsible for the blue coloration of sapphire, greatly intensify the color of the stone.  This treatment is now very widespread and more or less reproduces the sequence of events that occurred when many sapphire crystals were formed.  As a result, it is not always possible to distinguish a completely natural sapphire from one whose color has been intensified in this way, and they are treated as one on the market.  According to another procedure, however, colorless, pale yellow or pale green stones are covered in a paste consisting of iron and –mainly- titanium compounds.  The specimens are then heated to a temperature of about 1700 degrees Celsius for perhaps several days.  The iron titanium oxides slowly infiltrate the stones to a depth of about one millimeter, producing a deep blue coloration.  The stone then has to be repolished.