Gem minerals

A gem mineral is any mineral species that yields a gem upon cutting and/or polishing. Gem minerals have value based on their potential to produce gems. Although gems have had various uses in the past, including special powers attributed by folklore, their principal uses have been personal adornment (as jewels), a mode of investment, and a symbol of wealth and power.

Physical Properties

Gem minerals are those mineral species that have yielded the material from which specimens have been fashioned into gems. Gems and therefore gem mineral varieties must have the same properties. These properties are beauty (color, phenomenon, or clarity), durability (hardness and toughness), and value (rarity, demand, and tradition). Of the approximately five thousand mineral species known, only about ninety of these produce material in a quality and quantity suitable for gems, and this quantity of suitable material makes up generally only a very small portion of the gem mineral found. Often the quality of this gem material is such that the material is given a gem varietal name or mineral varietal name based upon color or phenomenon. A different name is given for each color variation; hence, there is a large number of gem names compared with the gem mineral names. Two or more different gem minerals may have the same gem name, as in the case of jade and the minerals nephrite (actinolite and tremolite) and jadeite. Finally, new gem varieties are found from time to time. For example, tanzanite was found in the 1960s and tsavorite during the 1970s.

Beauty

The beauty of a gem may be inherent in the gem mineral species—such as clarity, color, or phenomena (stars, eyes, and the like)—or it may be brought out as it is fashioned by cutting and polishing. Faceting is the cutting of a stone to add faces (facets) or flat surfaces, generally with a regular geometric form. Because faceting enhances the brilliance of a stone by causing reflections of both front and back facets, those mineral varieties having superior clarity became important gem varieties. The faceting of a clear gem mineral produces a color phenomenon called dispersion. This is the ability of a particular gem mineral to break light up into a rainbow of component colors, as does a prism. This property is commonly known as “fire” and is well known in diamonds.

Color is a very important property of gems. There are many gem varieties of gem minerals based solely on this property. For example, the named color varieties of beryl include aquamarine (blue-green), chrysolite (yellow-green), emerald (intense green), heliodor (brownish-green), and morganite (pink or orange). Clarity of color determines much of the beauty and value of gemstones. A “play of color” is produced by various combinations of reflection, refraction, diffraction, and interference phenomena; it is well known from such gems as opal (opalescence) and labradorite (labradorescence). Some stones are selected entirely for their color. These may be opaque or nearly so and may or may not take a high polish. They include such stones as turquoise, jade, and many varieties of quartz, such as jasper, bloodstone, agate, and carnelian.

Some stones have inclusions that produce aesthetic effects, such as stars, eyes, spangling, or even pictures. Stars and eyes are produced by minute aligned voids or needles of minerals like rutile. If they are in one direction, they produce eyes, as in chrysoberyl and tourmaline. If they are in a hexagonal or in other star-shaped patterns, they produce stars, as in star rubies, sapphires, diopside, and garnet. Spangling results from variously oriented small inclusions of reflective minerals within the gem mineral, such as mica, rutile, or tourmaline.

Durability

Durability of a gem mineral is its resistance to scratching or breaking, a necessity for an owner of jewelry because one does not want to damage a gem and thus markedly reduce its value simply by wearing it.

Hardness is the ability of a gem mineral to resist scratching. Mineralogists and gemologists rank minerals against a hardness scale of common minerals, called the Mohs hardness scale. From hardest to softest, the scale is 10 (diamond), 9 (corundum), 8 (topaz), 7 (quartz), 6 (orthoclase feldspar), 5 (apatite), 4 (fluorite), 3 (calcite), 2 (gypsum), and 1 (talc). The hardness of 7, the hardness of quartz, a very common mineral, is very important in the selection of gems or gem minerals. Quartz is a very common mineral, which means a stone with a hardness of less than 7 becomes easily scratched in our environment and therefore lacks durability. Most of our well-known gem minerals have a hardness greater than or equal to 7.

Resistance to breaking is referred to as tenacity. Most minerals are brittle, but native gold is malleable, ductile, and sectile.

Minerals break in two major ways called cleavage and fracture. Cleavage is the breaking of minerals parallel to more weakly bonded atomic planes found within the mineral and produces flat reflecting surfaces. Fracture is the random breaking of a mineral across atomic planes. Fracture along curved surfaces is called conchoidal fracture. Tenacity is directly related to the bond strength between adjacent atoms. Cleavage thus reduces tenacity, as in easily cleaved topaz, while an intergrown mat of fine crystals increases tenacity of a specimen, as in the case of the jades nephrite and jadeite. Cleaving was probably the first way to facet gems and still is very important in the shaping of rough diamonds before cutting and polishing.

Value

The value of a gemstone or a particular gem is based upon the interaction of many factors that can be divided into four major groups: beauty, rarity, quality of fashioning, and demand. Beauty includes such properties as color, quality of color, and clarity (freedom of inclusions, fractures, or cloudiness); it also includes phenomena such as quality of stars, eyes, or play of color. Rarity involves not only the rarity of the variety of the gem mineral but also the rarity of that quality of stone being used. For example, size influences the value because large stones are rarer than small stones; perfection influences the value because high-quality stones are less common than flawed stones. Rarity of the gem mineral can increase value or decrease value. Very common varieties of quartz, for example, may have less value than similar qualities and sizes of corundum because corundum is less common. Some gems, however, are so rare that people have not heard of them, and there is no demand except by connoisseurs and hobbyists, which in turn decreases value. Quality of fashioning includes degrees of perfection of symmetry and customizing of facet junction angles to the properties of the gem mineral species, which maximizes brilliance, color, and dispersion, and fineness or degree of polishing. Demand includes such issues as fashion, fads, and engagement ring and birthstone traditions. These associations, in general, increase value. Good or bad luck attributed to gemstones may increase or decrease their value, respectively.

The terms “precious” and “semiprecious” used with a gemstone name are misleading in assessing the value of the stone. “Precious” has been historically applied to gems such as diamonds, rubies, sapphires, opals, and emeralds, while “semiprecious” has been applied to such gems as aquamarine, tourmaline, chrysoberyl, citrine, topaz, and amethyst. Almost any gem mineral is found in a wide variety of qualities and therefore so are gems; thus, there are high-quality stones called semiprecious that are often more valuable than low-quality stones called precious. Unfortunately, the term “precious” has also been applied all too often to low-quality diamond, ruby, sapphire, opal, or emerald in order to deceive the buyer.

Formation of Gem Minerals

Gem minerals originate in the same way as all other minerals. They occur in four broad categories: igneous deposits (formed from molten rock—lava and magma), hydrothermal and pneumatolytic deposits (formed from hot water and steam, respectively), metamorphic deposits (formed by heat, pressure, and chemically active fluids with crystallization in the solid state), and sedimentary deposits (formed by processes of weathering, erosion, and deposition). Corundum is an example of a gem mineral that may be found under several conditions. A well-known igneous gem mineral formed from the cooling of molten rock is olivine (peridot), which is found in lava. Gem minerals found in plutonic rock (molten rock cooled within the earth) are diamond, various garnets, zircon, corundum, spinel, labradorite, orthoclase, and albite-oligoclase. Well-known gem minerals formed by hydrothermal and pneumatolitic processes are varieties of quartz (amethyst, citrine, crystal, smoky), varieties of beryl (aquamarine, morganite, emerald), various kinds of tourmaline, topaz, spodumene, and microcline. These minerals are commonly found in bodies or rock called pegmatites, or veins. Metamorphic rocks containing gem minerals form in the roots of mountain chains and/or adjacent to igneous bodies, where there are high pressures and temperatures. Well-known gem minerals formed here include corundum, kyanite, lazurite (lapis lazuli), various garnets, jade (both nephrite and jadeite), spinel, beryl, and chrysoberyl. Gem minerals found in sedimentary deposits may be divided into two major divisions: placer deposits (gravels that contain heavy minerals resistant to weathering and left by erosion and deposition of streams) and primary sedimentary minerals grown in the sediments. Placer gem minerals include diamond, corundum, spinel, tourmaline, topaz, various garnets, zircon, and chrysoberyl and may be associated with placer gold. Primary sedimentary minerals are mainly varieties of quartz (agate, chalcedony, onyx, sard, carnelian, petrified wood, heliotrope, bloodstone, jasper). Gem minerals may also result from weathering processes. A fine example is turquoise, which is produced by the weathering of copper-bearing hydrothermal veins under arid conditions.

Tools of the Gemologist

The binocular microscope is one of the most useful tools to the gemologist, whether for making identifications, grading, or preparing a stone for lapidary work. These microscopes have relatively low magnification, but the twin eyepieces allow the gemologist to view the stone in three dimensions. Other magnifying instruments used include the hand lens and loupe, which are mostly used for preliminary inspections of gems. Properties of minerals that can be determined for identification purposes with the binocular microscope include hardness—by noting polish quality or wear characteristics—and crystallography, which can be determined from traces of cleavage on the unpolished girdle of the stone, noting single or double refraction, orientation of inclusions, and other qualities. Properties of minerals that can be determined for the grading of gems include clarity, quality of cutting and polishing, symmetry, and color. Properties of minerals that can be determined for lapidary purposes include crystallographic orientation, cleavage directions, and location of color variations and inclusions. Most gems are considered flawless when no inclusions, fractures, surface blemishes, or cutting flaws are visible to the naked eye.

The refractometer is also one of the most useful tools of the gemologist for purposes of gem identification. This technique makes use of the fact that each mineral species has a distinctive and characteristic index of refraction, or indices of refraction. Different minerals may have one, two, or three indices of refraction. A refractometer such as the Rayner or Duplex refractometer can measure the indices of refraction of a gem by use of the critical angle of light for that mineral, which is dependent upon the mineral’s refractive index or indices. The index or indices of refraction are measured from flat facets of cut gems or crystal faces; an average of the indices may be obtained from a curved face or from any polished surface of a fine-grained aggregate, such as jade or chalcedony. Tables of refractive indices allow gemologists to compare results of their testing with those of known gem materials.

The polariscope is a simple yet very valuable instrument. It is a light source with two Polaroid plates mounted above and below the gem. The polariscope is used to distinguish between crystalline double refractive material (minerals whose optical properties vary with direction in the mineral) and singly refractive crystalline material or amorphous materials, and to identify doubly refractive fine-grained aggregates, such as jades and chalcedony. For example, ruby is doubly refractive and will change appearance when rotated in polarized light, whereas a similarly colored garnet will not.

The spectroscope may on occasion be a useful instrument. The hand-held types are difficult to use, and the table models are very expensive. They are generally not necessary for the identification of stones but are useful in confirming or determining the presence of metallic ions like iron or chromium. Light traveling through the mineral is separated into its color components in a rainbow of color by prism or diffraction. There will be dark lines on this rainbow, corresponding to light frequencies absorbed by atoms in the mineral. Genuine ruby will show absorption lines for chromium; red garnet does not.

Role in Economics and Technology

The study of gems or gem minerals interests the connoisseur, hobbyist, investor, and those whose business is the gem trade. Two reasons for a large public ownership of gemstones in Western society are the traditional diamond engagement ring in North America and the European use of the diamond as an investment. It is particularly in this market that most of the fraudulent practices in the gem and gem mineral trade exist. Investing in gems and gem minerals has been common for several reasons, including short-term and long-term gains, and as a hedge against inflation. Gemstones have also been valued for their portability, especially in unstable political situations. As with any investment, however, there is an element of risk. The degree of risk may vary with the gem mineral; for example, finding a large new deposit drops the price. Cornering of the market by a particular group forces the price up or down for some political or economic reason. Diamond pricing is probably the most structured and stable because it has been controlled by the De Beers Central Selling Organization (CSO) in London for more than a century. In 2023, De Beers set the price of a one-carat diamond at US$71. Many of the African nations where diamonds are mined are prone to civil war, and diamonds are frequently used to fund those wars. Such diamonds, called conflict diamonds or blood diamonds, are a major humanitarian problem because they are often mined with forced labor and battles are frequently fought for control of mining areas. Many diamonds are now engraved by laser with serial numbers that can be used to verify they did not originate in conflict zones.

Because of their perfection, gems are used by scientists for research concerned with crystals and crystallization processes. They also have many technological applications, from ruby (corundum) lasers to quartz crystals in citizen-band radios and telephone communications equipment. Because of the large demand for these minerals, synthetic substitutes are generally utilized today for their technological applications. Mineral species, of which the gem-quality varieties may make up only a very small quantity, may also have other major non-gem-related industrial uses. The mineral diamond, for example, has use as a gem not only in transparent, attractively colored, and colorless varieties but also in translucent, nearly opaque, or highly included stones, which are not pleasing to the eye yet have important industrial cutting and abrasive uses. Much high-technology grinding, cutting, and polishing is based upon diamonds, from the bits that drill oil wells and cut metals and the saws that cut stone to the fine abrasives used to polish gems and lenses. The mineral corundum, the second hardest mineral known, not only produces specimens of ruby and sapphire but is also extensively used as an abrasive. Some of the abrasive uses, such as coarse grinding with emery, have decreased because of the introduction of relatively inexpensive artificial diamonds, but other uses, such as polishing with alumina, have increased. The mineral hematite is best known as the chief ore of iron, but it is also used for black gems and has become more fashionable in recent years. The jewelry trade knows it best for yet another use: jewelers’ rouge, a finely powdered hematite used to polish metals and some gems. The gem mineral quartz, with many gem varieties—amethyst, citrine, smoky, rose, crystal, agate, carnelian, bloodstone—has many nongem uses, including glassmaking and the production of abrasives (sandpaper) and sand for sandboxes, beaches, and concrete.

Principal Terms

gem: a cut and polished stone that possesses durability, rarity, and beauty necessary for use in jewelry and, therefore, of value

gemstone: any rock, mineral, or natural material that has the potential for use as personal adornment or ornament

inclusion: a foreign substance enclosed within a mineral; often very small mineral grains and cavities filled with liquid or gas; a cavity with liquid, a gas bubble, and a crystal is called a three-phase inclusion

mineral: a naturally occurring inorganic substance with a characteristic chemical composition and atomic structure, manifested in its external geometry and other physical properties

mineral species: a mineralogic division in which all the varieties in any one species have the same basic physical and chemical properties

mineral variety: a division of a mineral species based upon color, type of optical phenomenon, or other distinguishing characteristics of appearance

rough: gem mineral material of suitable quality to be used for fashioning gemstones

synthetic mineral: a human-made reproduction of the structure, composition, and properties of a particular mineral

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