James Clarke Inc Stone Science

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Stone History
Stone Science
Diamonds
Colored Stones
The 4 "C's"

Gemstones have a wide diversity of qualities that can be described as beauty. The definition of that beauty is far from scientific; it tends to be defined by preference rather than science.

For gems, the first aspect of beauty is color, which is an outward manifestation of light. The deep green of an emerald or shimmering fire of an opal is produced by rays of light falling on the stone. The human eye perceives light wave lengths as colors. As the waves enter a stone as white light, they pass through the stone and are affected by the chemical and structural properties of the stone. Some light waves are absorbed, others are reflected. So a combination of different wave lengths radiates from the stone changing the white light that entered into colored light.

The chemical composition of the stone is not only responsible for its ability to absorb or reflect light, it is also responsible for the color. For example, a trace of chromium makes a beryl an emerald green. Chromium in a corundum makes a ruby red, and titanium in a corundum makes a sapphire blue, however, both gems share the same basic composition. Mineral composition is also responsible for unique light phenomena such as chatogancy or "cat's eye" effect, asterism (star formation), adulariscence (moonstone effect) and opalescence.

The scientific theory is that light is reflected by microscopically fine arrays of hollow tubes or mineral fibers. So if a stone is cut to create a surface above these reflecting inclusions, the broad sheen of reflected light becomes concentrated into a ray of light that floats over the surface and when the stone is moved, the image it creates moves too. This is the cat's eye effect. Similarly, asterism or star effect (as in star sapphires) comes from a multiplicity of parallel tubes or fibers crossing each other at angles of 90-120 degrees, thus creating a 4 or 6 rayed star.

The sparkle of a well cut ruby is apparent even to a layman. There is simply no way to duplicate the luster and sparkle that characterizes a gemstone. This is the result of several optical properties peculiar to gemstones, primarily refraction, double refraction, dispersion, luster, brilliance, and "fire." When a ray of light enters a gemstone, the velocity at which the light travels is affected by the dense packing of atoms. This causes the refraction of light because the ray of light is deflected from its original path by the denser medium. The angle of deflection or "index of refraction," as measured by instruments is one of the important diagnostic tools used by gemologists in identification. Generally speaking, the higher the refractive index (the slower the speed of light in the stone in relation to its velocity in air), the greater sparkle of the gemstone.

In precious stones, the refraction differs. This phenomenon is called double refraction. In some gemstones such as zircon, the double refraction is so high that the facet edges of the stone can appear doubled when viewed through the table of the cut stone.

Nor is the degree of refraction identical for all wave lengths. A particular crystal will impose a different speed on the red portion of the daylight spectrum than it does on the blue. This property differs from mineral to mineral and can be measured as "dispersion." This is also used in gem analysis. Dispersion has the effect of separating the colors. It accounts for the peculiar "fire" feature of diamonds. As light enters the stone, dispersion splits the light into its components as the colors of the spectrum.

Luster is the ability of the mineral to reflect light from the surface without allowing it to penetrate and contributes to the stone's sparkle. Reflection from penetrated light, described as brilliance, plays a much greater role. A clever design for the cut and optimal arrangement of the facets can cause the greatest possible amount of light striking the stone to be deflected in such a way as to reappear visibly at the surface. The index of refraction is a key consideration when cutting a stone.

The beauty and character of a gemstone is not determined only by its color or "fire," because inclusions (the presence of foreign matter) are of key importance. The presence of inclusions is very important in evaluating most gems. Inclusions can not only tell us something of the origin of the stone, it can also provide us with a window into geologic history. Most gemstones still carry the marks of enormous geological processes which go back much farther than the earliest artifacts we admire so greatly.

Inclusions occur through elimination or settling of foreign components in the host solution through developmental imperfections through chemical and/or physical changes in the process of crystallization, or for other causes. Depending on their characteristics, we can distinguish among solid, liquid and gaseous inclusions. It can be assumed that every gemstone contains a clue to its origins in the form of one or more inclusions even though present technology cannot always find them. Solid inclusions generally represent foreign minerals. Frequently the faces of fully formed tiny crystals can be seen under the microscope. Liquid inclusions are the most beautiful, and to a gemologist, the most interesting. Depending on their shape, they are described as flags or cavities. Cavities in which the outer crystal faces follow the shape of the host crystal are called negative crystals. Many of them contain the residue of a million year old mother liquid, such as water, carbonic acid or salt solutions. The three-phase inclusions – in which a liquid filled cavity holds both a gas bubble and a small crystal – are particularly interesting.

An inclusion need not be considered a flaw, they can be a clue to the origin and history of the stone, and they can be a source of identification.

Gemstones are rare. Their rarity is related to their development rather than to their composition because most are composed of chemicals found in abundance throughout the world (silica, carbon, aluminum and lime, etc.) Most gemstones are minerals, but some are rocks. Minerals are physiochemically uniform, structurally separate, natural components of the earth's crust. Rocks are aggregates of disparate solids belonging to one or more mineral varieties.

Most gemstones are crystals – that is they are structured matter in which an ordered array of atoms chemically bond together to form a particular crystalline structure. The mutual attraction of these electrically charged atoms, called ions, results in a bonding called cohesion. Special conditions are necessary in nature for ions with opposite charges and which are complementary to each other to appear in gasses or molten liquids. Further, the proper conditions of pressure and heat must occur to enable them to form atomic aggregates of space filling lattices whose scheme and structure are expressed outwardly in a symmetrical arrangement of multiple faces, the crystal.

Although it is true that every mineral has a particular internal structure, some elements are able to join together in more than one way. Thus, two minerals with totally different properties may have exactly the same chemical composition. Minerals of this type are known as polymorphs (many forms). Graphite and diamond are particularly good examples of polymorphism because they consist exclusively of carbon yet are drastically different. Graphite is the soft gray material of which pencil lead is made, while diamond is the hardest known mineral. The differences between these minerals can be attributed to the conditions under which they formed. Diamonds are believed to form at depths approaching 200 kilometers where extreme pressures produce the compact structure. Graphite, on the other hand, consists of sheets of carbon atoms that are widely spaced and weakly held together. Since these carbon sheets will slide easily past one another, graphite makes an excellent lubricant.

Three main cycles of rock and mineral formations are:

1. the magmatic cycle (magma = viscous mass)

2. the metamorphic cycle (metamorphosis = change of state)

3. the sedimentary cycle (sedimentum = deposit)

The magmatic cycle is named for the molten rock material within the earth, the magma. Magmatic minerals were formed either at some depth in the earth or in eruptions near the surface. The solidification of the magma and crystallization into minerals is an extremely complex process lasting millions of years.

In the first stage of the magmatic cycle, the liquid magmatic phase at temperatures from 1,500 to 700 degrees centigrade, the primary formation of rocks occurred. The igneous rocks (granite, dolerite, gabbro) were formed at this phase. With the exception of the diamond, only microscopically small quantity of gemstones were formed, so generally commercial value of magmatic gemstone deposits is small.

Further cooling of the magma left residual molten matter in which the volatile constituents became increasingly concentrated: water, boron, chlorine, fluorine, carbonic acid, phosphorus, sulfur, as well as a number of rare elements. There was considerable pressure within this residue, and the residual solutions oozing from the solidifying magma created a further rise in pressure, until the liquids forced their way into cracks and fissures where the chemical and thermal effects changed. In this pegmatitic phase (pegma = texture or framework) the most beautiful and the largest gem crystals were formed. The quantity of water in these residual fluids, still at temperatures between 700 and 500 degrees centigrade, rendered them particularly motile, and the abundance of volatile residual constituents favored the formation of large crystals. It was this phase which gave rise to apatite, beryl, chrysoberyl, euclase, kunzite, moonstone, sapphire, spodumene, topaz, tourmaline and many others.

As the molten rock mass continued to cool, it was further enriched with the additional volatile constituents. New solutions containing gas and steam penetrated even more deeply into the surrounding strata causing chemical alterations. The pneumatolytic deposits (pneuma = breath, lyein = to dissolve) emerged during this phase. While the solutions were still hot (500-400 degrees centigrade), their chemical interaction with the adjacent rocks formed new and stable minerals. The extremely potent chemical activity and the motility of these residual solution resulted in mineral assemblages. The great variety of gemstones formed in this way (Burma, Thailand, Urals and in other areas) include alexandrite, ruby, sapphire, spinel, emerald and lapis lazuli.

The last part of the magmatic cycle was the hydrothermal phase (hydro = water, thermos = hot). While temperatures ranged from 400-100 degrees centigrade, the watery solutions of those elements that so far had formed no compounds were under sufficient pressure to extrude through cracks and fissures, crystallize as cooling continued, and fill cavities in the rocks. This phase accounts for the formation of the large family of quartzes, as well as the tourmalines and topazes. The emeralds of the calcareous shales of Columbia are also of hydrothermal origin.

The metamorphic cycle has little to do with gem formation. Thrusting, faulting and circulation of residual solutions subjected rocks already existing in the magmatic phase to increased pressure or temperatures. Previously formed rocks and minerals evolved into new minerals of a different kind. Almandine, jadeite and nephrite developed during this phase.

The sedimentary cycle was the last cycle of rock formation. It is also of minimal importance for the creation of gemstones. When formation has been completed, weathered rock, exposed to water and wind, can accumulate dry deposits which in turn become compressed. An interesting ornamental stone of this sedimentary cycle is rhodochrosite, found as stalagmites in San Luis, Argentina. Wind and weather can transport previously formed rocks and minerals from the original site of their formation to other sites, sometimes at a great distance, where they can be found concentrated as secondary deposits. Turquoise, chrysoprase, malachite and azurite are formed from sedimentary weathering.

It is hard to believe that a precious stone today will still be as beautiful, colorful, and sparkling thousands of years from today. Because of their ability to withstand such physical attacks as abrasion, scratching, corrosion, chemicals and the effects of light, gemstones are resistant to processes that might diminish their beauty.

Additional Detailed Gem Science Information

It was only in this century that stones were classified into various categories such as precious stones, semiprecious stones and ornamental stones. In 1880, for example, the diamond, corundum, ruby, emerald, sapphire, amethyst, agate, aventurine, garnet, lapis lazuli, opal, topaz and turquoise were all classified as precious stones. Well into the 20th century the preference was to divide the minerals used in jewelry into two categories, gems (when mounted in jewelry) and ornamental stones (carvings, mosaics, inlays, etc); the term precious stone is strictly reserved for diamonds, rubies, sapphires and emeralds.

Modern classification separates minerals into several categories:

PURE ELEMENTS: Diamonds (See separate page devoted to diamonds)
SULFIDES: Iron Pyrite and Chalcopyrite, a cut form "marcasite" is used in jewelry.
HALIDES: Fluorite, used as an ornamental stone because it is very brittle.
OXIDES: See below.
CARBONATES: See below
SILICATES: See below
PHOSPHATES: See below
NON-CRYSTALLINE and ORGANIC MINERALS: See below
COMMERCIALLY CREATED AND FALSE STONES: See below

OXIDES:
Chrysoberyl (beryllium aluminum oxide). Generally yellow in color, the chrysoberyl has two particularly interesting varieties: cat´s eye or chymophane, in which the cabochon cut brings out a special iridescence in the form of a luminous silvery band between two colored zones, yellow and pale green, and the very rare and valuable alexandrite. Discovered in 1831 at Tokowaya in the Urals, the alexandrite is emerald green in daylight but dark red in artificial light.

It occurs in acid rocks or where they are in contact with micaschists or dolomitic marbles and in alluvium, The best alexandrites come from the Urals; large stones are also found in Ceylon, together with cat´s eye, and yellowish-green and pale green-yellow chrysoberyl. Some other sources are Africa and Brazil and Sri Lanka. A good quality Russian alexandrite can be more valuable than a sapphire.

Corundum (aluminum oxide). Known in India from the beginning of historic times, they occur in a wide range of colors from rosy-pink to the dark red (ruby), from blue and violet (sapphire). With the exception of the ruby, all of the colored varieties are commonly given the name of sapphire, whether white, golden, pink or green. Unlike the diamond, the corundum does not diminish in value when small inclusions are present, provided they are not too visible.

As with other gems, the inclusions can help to determine the natural stones from valueless synthetic corundum. Corundums sometimes exhibit the phenomenon of asterism (the six rayed star). This is caused by the presence in the stone of a lattice of fine needles of rutile. Star rubies and sapphires are always cut in cabochons so as to show the star to its best advantage. It appears to shimmer across the surface of the stone with the least movement. Although there are many sources of corundum, the best stones are found in pockets in volcanic rocks and in the re-crystallized calcite of Burma, and the alluvium of Sri Lanka and Thailand.

Hematite (iron oxide). Known for thousands of years, hematite is used for inexpensive jewelry because of its fine metallic sheen ranging from black to steel-grey. It is a common mineral found in many different types of deposits; the finest crystals come from Brazil and the island of Elba.

Quartz (silicon oxide).: One of the first minerals used by prehistoric man, quartz has a wide range of colors. It also has a wide range of names: rock crystal (colorless), amethyst (violet), citrine (yellow), as well as rose, blue, smoky and milky quartz, all of which are crystalline forms. Aventurine contains sparkling scales of brown mica, and other stones contain needle-like crystals of tourmaline or rutile (Venus hair stone). Tiger's eye contains crocidolite, and can be old gold or blue, chatoyant (iridescent) or opaque. So-called cryptocrystalline quartz contains microscopic crystals and is known in many different forms with the general name of chalcedony.

Different chalcedonies can be red (carnelian), apple green (chrysoprase), dark green with whitish or yellowish spots (plasma), brownish red (sard) or dark green with red spots (bloodstone or heliotrope). White and blue and gray forms are also common. Agate is a chalcedony banded in differently colored concentric zones. Onyx is agate with alternative black and white bands, either parallel or concentric. Sardonyx has alternate red-brown and white bands; moss agate contains red and green inclusions with a leafy look. Jaspers are chalcedonies containing as much as 25% impurity, and a wide variety of color. The red spots in bloodstone are jaspers.

Quartz is one of the most widespread of minerals. It is found in the acid igneous rocks, in pegmatites, in hydrothermal lodes, and in crystalline schists. Chalcedonies are of secondary origin because they were formed as crusts and concentrations in the sources of thermal springs or as amygdules in basic volcanic rock.

Opals are a hydrated form of silicon oxide with an amorphous structure. One of the most beautiful gems, the opal is translucent and bathed in a milky light or `opalescence´ shot with lively spectral colors. This opalescence is caused by the many changes of refractive index produced by the thin layers of stone and air, and according to the angle of the cross-section. The colors can range from white, orange and red to dark blue (black opal). The finest opals come from Australia, Hungary and Mexico. The Mexican fire opal has a red or orange color. Opals can be found as nodules or encrustations in volcanic rocks such as andesite or trachyte.

Spinel (magnesium aluminum oxide) is a trickster because the finest colored stones have often been mistaken for rubies or sapphires. Colors include red, pink, violet, orange, blue and brown. The best stones are found in Burma and India.

CARBONATES:
Smithsonite (zinc carbonate) can be pale yellow or green, may be cut and faceted, but are rarely not brilliant stones; however, the translucent blue-turquoise is very beautiful when cut as a cabochon. Smithsonite is very rarely of gemstone quality. It is found in weathered parts of zinc ores or in substitution masses in limestone.

Rhodochrosite (maganese carbonate) has been used in jewelry because of its red masses and pink highlights. IT can be found with various manganese ores and in certain substitution deposits in limestones.

Azurite (copper carbonate) is a beautiful deep blue stone. Complexes of azurites and malachites are particularly sought after because of the rich contrasts of color. The mineral is found in upper copper levels.

Malachite (copper carbonate) is another stone that has been known since ancient times. It is used for ornament because of its rich green zones of lighter color. Very beautiful objects can be cut from this stone. It is widespread with large quantities found in Siberia and like azurite, is found in upper levels of copper deposits.

PHOSPHATES:
Brazilianite (sodium aluminum phosphate) is one of the modern gems, because it was described for the first time in 1945. Beautiful transparent green-yellow crystals are found in the pegmatites of Brazil and the USA.

Apatite (calcium phosphate containing fluoride and chloride) is quite widespread and is found in hydrothermal lodes, igneous and metamorphic rocks as crystals that can be colorless, yellow, green, pink or violet.

Variscite (hydrated aluminum phosphate) is also known as Utahlite. It occurs as compact masses of opaque greenish blue with veins of brown. Like malachite, it is usually cut into objects. A rare mineral, it is of secondary formation and is found in desert regions. The Egyptians mined it on Sinai, the Persians at Khorassan and it was also worked by the Mayan, Inca and Chimu.

SILICATES:
Phenkite (beryllium orthosilicate) is found in crystalline form, usually colorless, pale pink or yellowish. It is also found in pegmatites, granites and crystalline schists. The stones are valued by collectors of rare gems.

Oviline (iron magnesium orthosilicate), the name is often wrongly applied in English to green demantoid garnet. The gem variety of iron magnesium silicate is usually called peridot, the name chrysolite formerly applied to the yellowish-green variety of this stone is better avoided. In ancient times, peridot was confused with topaz. Olivine is a widespread mineral, but the gemstones which may be found in very basic rocks are becoming rare.

This group of complex silicates, crystallizing in the cubic system forms an interesting family of gems. Pyrope (magnesium aluminum silicate) is of a good red color and is often known as Cape Ruby or Arizona Ruby. It is found in igneous rocks such as peridotites, kimberlite or serpentine. Almandine (iron aluminum silicate) is probably the crusaders carbuncle. It is deep red and is found in gneiss and micaschist. Spessartite (manganese aluminum silicate) is orange-red or brown. It is rare and is found in granites, pegmatites and rhyolites. Gossular garnet (calcium aluminum silicate) can be brownish yellow (cinnamon-stone), reddish-orange (jacinth), green (Transvaal jade), or other colors. It is found in the metamorphic limestones.

The most sought after demantoid garnet or olivine is a verity of andradite (calcium iron silicate). It is green and like the diamond shows brilliant refraction colors. It is found in the contact zones of metamorphic strata, serpentine and chlorite schist in the mountains of Italy and the Urals. Uvarovite (calcium chromium silicate) also comes from Russia. It is bright green, often opaque and is found in chromium bearing serpentine and metamorphic limestones.

Zircon (zirconium orthosilicate.) The normal or high type occurs in a wide range of colors: yellow, light green, blue and red. The blue and red are sometimes known as hyacinth. In the low type, the crystalline structure is almost completely broken down into an amorphous sate. It is greenish in color. Stones of this type are found mostly in Sri Lanka; they can be converted into normal crystalline type by prolonged heating. Zircons are found in acid igneous rocks, pegmatites and nephelinic syenites.

Euclase (Beryllium aluminum subsilicate). This stone has become so rare that many collector prefer to keep it uncut. Pale green, pale blue or colorless, it is sometimes found in pegmatites.

Andalusite (aluminum subsilicate). This stone can sometimes occur as fine green or red-brown stones with changing tones. It is found in gneiss, micaschist and pegmatites.

Kyanite (aluminum subsilicate). Found in gneiss or micaschists, this stone has a fine sky blue or blue green color.

Topaz (aluminum fluoro-silicate). Topaz is found as large fine stones which may be colorless, blue, yellow or orange. Pink stones are almost always produced by "firing" or heat treating the stone to change its color. Topaz occurs in acid igneous rocks in Brazil, Siberia and the USA. Topaz crystals can be huge, some weighing over 100 kilograms.

Sphene or Titanite (calcium titanium subsilicate). Well cut stones which can be yellow-green or pinkish-brown, have a sharply colored brilliance. They are found in igneous or metamorphic rocks in Switzerland, Mexico and Burma.

Dumortierite (aluminum boron hydroxysilicate). This stone is increasingly used for ornament because of its fine blue-violet color. It can be found in California, Arizona and Nevada. It is a metamorphic mineral found in association with acid rocks or in seams of quartz.

Zoisite (calcium aluminum hydrosysilicate). Rose-red, green or brown, this stone occurs igneous rocks with plagioclase feldspar, or in crystalline schists.

Prehnite (calcium aluminum acid silicate). Green, jade like or with a gray or brownish tinge. A mineral of secondary origin, it is found in granites, gneiss, micaschists and basic volcanic rocks in France, the USA and Australia.

Benitoite (barium titanium silicate). A rare gem very much like a sapphire. Stones over three carats can command very high prices. Benitoite occurs in San Benito, California.

Beryl (beryllium aluminum metasilicate). While some crystalline stones can weigh as much as 100 tons, gem quality stones are rare. The emerald differs from other beryls in its deep green color, and has given its name to that luminous richness so sought after since ancient times. Most emeralds come from Columbia. In the mine at Muzo, crystalline emeralds are found in hydrothermal penetration of complex bituminous deposits and igneous rock intrusions. Some beautiful stones have been found in Transvall, India and Brazil.

Aquamarine is blue to blue-green, morganite is pink, heliodor is golden yellow; there are also colorless, pale green and yellowish green beryls. Gems are found in pegmatites and adjacent to granites, particularly in zones of metamorphic limestones. They are produced principally by Brazil and in regions of Minas Gerais and Bahia. Crystals weighing 20 kilos or more have yielded huge cut stones. The aquamarines found in the Ural mountains are a very beautiful blue; those found in Madagascar are light pink to violet pink morganites. Mozambique is known for fine beryls as well as tourmalines.

Iolite or Cordierite (magnesium iron alumino-silicate). A good blue stone, strongly pleochroic (deep blue/grey-blue/grey-yellow). It is found in certain volcanic rocks and lavas, and in some metamorphic rocks.

Tourmaline (complex borosilicate). Elbaite (lithium containing tourmaline) provides stones of marvelous colors ranging from pale pink, yellowish brown, green to greenish blue. Some crystals have two-toned green outer bands with red inner core, they are known as watermelon tourmaline because the color striation is similar to a halved watermelon. Rubellite is from pink to violet-red and good stones are ranked very highly. Indicolite is dark blue or blue-green.

Dravite (tourmaline continuing magnesium) is brown, sometimes with a yellowish tinge. Schorl contains iron and is black.

Dioptase (copper acid silicate). Always very beautiful deep emerald-green. Much sought after by collectors, this mineral occurs in the weathering zone of certain copper deposits.

Chrysocolla. A hydrous copper silicate with an amorphous structure. Turquoise-blue or green, with a glazed appearance. IT is found in weathered copper deposits.

Diopside (calcium magnesium metasilicate). Green or yellowish-green, sometimes white or brown in color. Violane is opaque violet-blue. A variety of diopside, black in color because of the presence of fine layers of magnetite shows a fine four-rayed asterism. Diopside is found in metamorphic rocks.

Spodumene (lithium aluminum metasilicate). Sometimes occurs in fine crystals with delicate colors ranging from yellow to pale green. Kunzite, first discovered near San Diego, California, is rose-colored; hiddenite from North Carolina is grass-green. Spodumenes are not as valuable as beryls of corresponding colors. They are found in granites and pegmatites.

Jade is a name given to a group of silicates with an opaque waxy, microcrystalline structure. There are two minerals which comprise true jade: jadeite, sodium aluminum silicate and nephrite, a complex hydrous silicate containing calcium, magnesium and iron, and closely related chemically and geologically to asbestos. Jadeite can be white, green, pink, mauve or brown. Particularly sought after is the emerald-green variety also known as Imperial Jade. It is found in the thin seams of serpentine rocks. Nephrite can be white, gray, a wide range of greens, and sometimes, pink. It can also be found in serpentine rocks, or in metamorphic schists. Chloromelanite is a variety of jadeite which is nearly black due to the inclusion of iron; chromojadite contains chromium.

Rhodonite (manganese metasilicate). A stone with a good pinkish-red color, it is found in some iron and manganese deposits; the most famous is at Franklin Furnace in New Jersey.

Antigorite (magnesium silicate) Also known as serpentine, antigorite is a green stone resulting from the weathering of magnesium silicates. It has been used as imitation jade.

Feldspars as a group are the most common of all minerals. Orthoclase and microcline are aluminum potassium silicate; oligoclase and laboradorite replace the potassium with varying amounts of sodium and calcium. Orthoclase is colorless or yellow. The moonstone is milky-white with a silvery opalescence. Microcline has a fine bright green from known as Amazon stone, can be found at Pike´s Peak in Colorado. It has been known since the earliest of times and can be found in syenites, granites and pegmatites.

Oligoclase is known as sunstone or aventurine and has a spangled brown color due to the inclusion of flakes of hematite or geothite. Labradorite occurs in gabbros, and is grey-blue to blackish blue or green with brilliantly colored iridescence.

Sodalite (complex sodium silicate containing chloride). It is found in syenites and soda lavas and is a beautiful deep blue. The stone is often used in jewelry and as an ornamental.

Lazurite (complex aluminum silicate) A metamorphic mineral which is a very beautiful blue. It is one of the chief constituents of lapis-lazuli. The crystals can be cabochon cut.

Lapis Lazuli is formed from many minerals principally lazurite but also minerals such as sodalite and hauynite and calcite or pyrite. Known and sought out for at least 6,000 years because of its rich blue color, lapis lazuli occurs rarely in masses of any importance. The oldest known source is in Afghanistan, where the mineral occurs in a marble inclusion in a metamorphic rock. Other localities in which lapis lazuli suitable for ornamentation is found include a site near Lake Baikal in Russia, the province of Coquimbo in Chile, and in California.

NON-CRYSTALLINE AND ORGANIC MINERALS
Amber is a fossil resin, yellow to yellowish-brown, greenish yellow or reddish. It becomes electrically charged on rubbing and burns with a pleasant smell. It is notable for its very low density, being only slightly heavier than water.

Jet is fossilized wood related to coal. It was particularly popular during the last century for the fine black sheen it has when polished.

Tektite. A naturally occurring glass, possibly of meteoric origin. The dark green or blue specimens are used in jewelry under the name of noldavite. It is found in recent Tertiary rocks and on the plains of central Europe, Australia and southeast Asia.

Obsidian. A dark brown or black volcanic glass sometimes with fine chatoyancy. The best obsidians came from Mexico, where they were magnificently worked by the Aztecs. Well known among the ancient peoples of the east, obsidian was also valued in Kenya and Abyssinia during the middle and upper Paleolithic for the manufacture of everyday objects.

Coral. The chalky skeleton of the coral polyps (colonies of tiny animals which proliferate in temperate and warm seas at depths between 50 and 200 meters). Rose colored and red corals have been used for a very long time in ornament and statues carved from large pieces of coral are very valuable.

Pearls. Roughly concentric accretions of an organic substance, conchiolin, and calcium carbonate in the crystalline from known as aragonite. The best pearls are spherical, although they can also be of irregular shape (baroque). They occur in certain salt or fresh water mollusks. Sought after since the most ancient times, pearls have a particular luster of their own. Colors can range from pink to yellow, from bluish to palest green with an infinite range of intermediate shades including white. There are blue, black and pink pearls, but the highest value are from the salt water mollusk Meleagrina. Cultured pearls are made by the artificial insertion of tiny balls of mother of pearl into the tissues of oysters. They are produced on an industrial scale and are much less valuable than natural pearls also known as fine pearls. Principal pearl fishing localities are the Persian Gulf, the Gulf of Mexico, Sri Lanka, northwest coast of Australia and numerous islands in the Pacific.

COMMERCIALLY CREATED AND FALSE STONES include glasses, synthetic gems, and doublets.

Lead glasses have been used to imitate gems since the 18th century. They have no gemological value, however the piece of jewelry might have historical value. Glasses also include enamels, which can have value, but that is another discussion.

Synthetic gems are made on an industrial scale like chemicals. These artificial stones bear the name of the mineral species they resemble because they have an identical atomic structure and a near (not always identical) chemical composition. The first commercial exploitation of the false gems began at the end of the 19th century. The stones are seldom of any value.

Doublets are easily detected. They are composite stones made up of two or sometimes three pieces (triplets) cemented together with a transparent cement to produce a larger stone. There are various combinations: gem on gem, gem on glass, gem on stone, etc. As in the previous cases these stones do not have much value.