Gemstone

• 1. Introduction

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  A gemstone is a piece of mineral crystal that, once cut or polished, becomes jewelry or adornment. The colored gemstone industry, meaning everything other than diamonds, is currently estimated at US$1.55 billion. It is projected to reach a value of $4.46 billion by 2033. Behind those figures sits a strange world of vocabulary and judgment. The gem expert is a gemologist. The gem maker is a lapidarist or gemcutter. The diamond cutter has a name of his own, the diamantaire. Why are emerald, ruby, sapphire and diamond called precious, while everything else is merely semi-precious? How does a colorless mineral become a green emerald or a pink stone? Why does a single gem earn one name from one laboratory and a different name from another? And how can a crystal grown in a furnace be chemically identical to one pulled from the earth? These are the questions that run beneath the surface of every polished stone.

• 2. Precious And Semi-Precious

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  The traditional Western classification reaches back to the ancient Greeks, beginning with a split between precious and semi-precious. In modern use the precious stones are emerald, ruby, sapphire and diamond, with everything else falling into the semi-precious category. That distinction reflected the rarity of those stones in ancient times, along with their quality. All are translucent, hold fine color in their purest forms apart from the colorless diamond, and rate a hardness of 8 to 10 on the Mohs scale. The traditional labels do not always track real value. Most garnets are relatively inexpensive, yet a green garnet called tsavorite can be worth far more than a mid-quality emerald. Art history and archaeology use another term for semi-precious stones, calling them hardstone. The commercial use of precious and semi-precious is arguably misleading, since it implies a value ranking that the market does not always honor. Some stones once held precious status and lost it. Amethyst was counted among the cardinal gems, a precious stone going back to ancient Greece, until bulk discoveries in Brazil in the 19th century changed its standing. Aquamarine, peridot and cat's eye, also called cymophane, were popular enough in the last century to be regarded as precious. The lesson is that rarity, more than any fixed quality, has long decided which stones earn the prestigious names.

• 3. Reading A Stone

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  Chemical composition is the first thing a gemologist examines. Diamonds are made of carbon, while sapphires and rubies are made of aluminium oxide. Many gems are crystals sorted by crystal system, whether cubic, trigonal or monoclinic, and described further by habit, the form in which the gem is usually found. Diamonds, with a cubic crystal system, often turn up as octahedrons. Groups, species and varieties organize the gem world in surprising ways. Ruby is the red variety of the species corundum, and any other color of corundum is sapphire. The species beryl spans emerald in green, aquamarine in blue, red beryl in red, goshenite when colorless, heliodor in yellow, and morganite in pink. Gemologists describe stones through a dense vocabulary of properties. They measure hue, tone and saturation of color, along with luster, refractive index, birefringence, dispersion, specific gravity, hardness, cleavage and fracture. A stone may show pleochroism, double refraction, luminescence, or a distinctive absorption spectrum. One older grading term is a gem's water, a measure of its luster, transparency or brilliance. The most transparent stones are called first water, while second or third water stones show lesser transparency. Material or flaws trapped inside a stone appear as inclusions, the tiny internal features that often reveal where a gem truly came from.

• 4. The Four Cs

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  Gemstones have no universally accepted grading system, which makes diamonds the exception. The Gemological Institute of America, or GIA, developed the diamond grading system in the early 1950s. Its major innovation was introducing 10x magnification as the standard for grading clarity. Other gemstones are still graded by the naked eye, assuming 20/20 vision, the same way all gems were once judged. The mnemonic of the four Cs, standing for color, cut, clarity and carats, was introduced to describe how a diamond is graded. The criteria carry different weights for colored stones than for colorless diamonds. In a diamond, cut is the primary determinant of value, followed by clarity and color. An ideally cut diamond breaks light into its rainbow colors through dispersion, chops it into bright pieces through scintillation, and delivers it to the eye as brilliance. In rough crystalline form a diamond does none of this, which is why fashioning it is called cut. For any stone that has color, including colored diamonds, the purity and beauty of that color becomes the primary determinant of quality. Colored stones bring their own quirks. Emeralds will always carry a number of inclusions, so clarity matters less. Value can rise from unusual optical phenomena such as color zoning, the uneven distribution of color within a gem, and asteria, the star effects that seem to float across a surface. Some buyers value gems for reasons outside beauty. Proponents of energy medicine prize them for their alleged healing powers, a belief that sits far from the gemologist's microscope.

• 5. Battle Of The Labs

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  A stone can be called pink by one laboratory and padparadscha by another. One lab may declare a gem untreated while a second concludes it has been heat-treated. Many laboratories grade gemstones and issue reports, each with its own methodology. The GIA is the main provider of education and diamond grading reports. The International Gemological Institute grades diamonds, jewelry and colored stones. The Hoge Raad Voor Diamant, or HRD Antwerp, the Diamond High Council in Belgium, is one of Europe's oldest, with the Antwerp World Diamond Centre as its main stakeholder. Others include the American Gemological Society, the American Gem Trade Laboratory within the American Gem Trade Association, and the American Gemological Laboratories owned by Christopher P. Smith. The European Gemological Laboratory was founded in 1974 by Guy Margel in Belgium. Across the world sit further institutions of note. Japan has the Gemmological Association of All Japan, known as GAAJ-ZENHOKYO. Thailand has the Gem and Jewelry Institute of Thailand in Bangkok. Africa has the Gemmology Institute of Southern Africa, and South East Asia has the Asian Institute of Gemological Sciences, the oldest in that region. The Swiss Gemmological Institute, or SSEF, was founded by Henry Hänni, while the Gübelin Gem Lab was founded by Eduard Gübelin. To narrow the gaps between verdicts, seven respected labs formed the Laboratory Manual Harmonisation Committee, or LMHC. These were AGTA-GTL in New York, CISGEM in Milano, GAAJ-ZENHOKYO in Tokyo, GIA in Carlsbad, GIT in Bangkok, Gübelin in Lucerne and SSEF in Basel. They work to standardize report wording and the interpretation of results. Country of origin remains hard to pin down, since new source locations keep being discovered. Gem dealers know the differences between labs and exploit them, shopping their stones around to obtain the best possible certificate.

• 6. Cut Polish And Color

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  Most gemstones are cut and polished before they ever reach a setting, though a few are used in the raw crystal forms in which they are found. The two main classifications divide the craft. Cabochons, sometimes shortened to cab, are smooth dome-shaped stones, popular since ancient times and more durable than faceted gems. Faceted stones are cut on a faceting machine that polishes small flat windows, the facets, at regular intervals and exact angles. The choice of cut follows the nature of the stone. Opaque or semi-opaque gems such as opal, turquoise and variscite are commonly cut as cabochons, shaped to show color, luster and surface properties rather than internal reflection. Transparent gems are normally faceted, a method that maximizes reflected light and produces the sparkle a viewer perceives. The facet angles must be exact. If they are too steep or too shallow, light passes straight through instead of bouncing back to the eye. Color itself is a trick of light and impurity. White daylight contains all the colors of the spectrum, and a material absorbs most of it while reflecting a particular wavelength. A ruby looks red because it absorbs every other color and reflects only red. Ruby and sapphire share the same composition as corundum, yet differ in color because of impurities that absorb and reflect different wavelengths. The atomic structure tells the deeper story. Now and then an atom is replaced by a different one, sometimes as few as one in a million. Pure beryl is colorless. Add chromium and it becomes emerald. Add manganese instead and it becomes pink morganite. Add iron and it becomes aquamarine.

• 7. The Art Of Enhancement

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  Foiling, in which metal foil is placed behind a stone to enhance its color, dates back to the Minoan Age, among the earliest gemstone treatments known. The Roman writer Pliny the Elder recorded oiling and dyeing or staining in his book Natural History some 2000 years ago. Treatment can improve color, clarity or even durability, but it can also spoil a stone and may affect its value depending on type and extent. Heat is the oldest workhorse of the trade, known to miners and cutters for centuries. Most citrine is made by heating amethyst, and partial heating with a strong gradient produces ametrine, a stone partly amethyst and partly citrine. Aquamarine is heated to remove yellow tones or deepen its blue. Nearly all tanzanite is heated at low temperatures to remove brown undertones, and much sapphire and ruby is heat-treated to improve color and clarity. Heat carries a warning for the bench jeweler. When jewelry with diamonds is heated for repairs, the diamond should be protected with boric acid, since it is pure carbon and could burn on the surface or even burn up completely. Sapphires and rubies should not be coated with boric acid, which can etch them, though they must still be shielded from heat stress fracture by immersing the stones in water during work. Radiation reaches colors that barely exist in nature. Virtually all blue topaz, including the darker London blue, has been irradiated to change from white to blue. Most green quartz, called Oro Verde, is irradiated for its yellow-green color. Light-to-medium-yellow diamonds may turn green under gamma rays, or blue under a high-energy electron beam. Done in a nuclear reactor, such processes can make gemstones radioactive, and the health risks have led to government regulations in many countries. Other treatments fill what nature left open. Emeralds with natural fissures are filled with colored wax or oil to disguise flaws and improve color, and turquoise is treated the same way. In 2006 glass-filled rubies drew publicity, as rubies over 10 carats with large fractures were filled with lead glass to improve their appearance, though such treatments are fairly easy to detect. Bleaching uses a chemical to reduce color, often followed by dyeing, and has notably been used on jade and pearls.

• 8. Grown In A Furnace

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  In 1837 the first successful synthesis of ruby occurred when French chemist Marc Gaudin produced small crystals by melting together potassium aluminium sulphate and potassium chromate, a method later known as the flux melt process. Another French chemist named Fremy grew large quantities of small ruby crystals using a lead flux. A so-called reconstructed ruby was then marketed as larger rubies made from melted bits of natural ruby, though later attempts found the process impossible, and reconstructed rubies were most likely made by melting ruby powder in several steps. The breakthrough came from Fremy's student. Auguste Verneuil developed the flame-fusion process in 1902, building large furnaces that produced corundum efficiently and shifted the gemstone market. The flame fusion process runs in a Verneuil furnace, where an inverted blowpipe burner makes an extremely hot oxyhydrogen flame. Chemical powder drops through the flame, melts, and solidifies on a pedestal into a crystal called a boule. For corundum the flame must reach 2000 °C. Pure aluminium powder takes on different additives for different colors, such as chromic oxide for ruby, iron and titanium oxide for blue sapphire, and cobalt for dark blue sapphire. Flame fusion remains the most cost-effective method, and world production of corundum this way reaches 1000 million carats a year. Other methods followed for other stones. In 1918 Jan Czochralski developed the crystal pulling method, in which a seed stone is dipped into melt and slowly pulled away as the gem grows. Flux growth, which first synthesized emerald, dissolves gem ingredients in molten flux inside a graphite or platinum crucible, taking two months up to a year. Hydrothermal growth seals materials in water under extreme pressure heated past its boiling point, finishing in a few weeks. These distinctions matter to the trade. Synthetic gems are physically, optically and chemically identical to natural stones, while imitations such as cubic zirconia, synthetic moissanite, glass or plastic merely look the part. Moissanite has a higher refractive index than diamond and shows more fire beside an equal stone. Lab-created stones often look more vivid, since the impurities that dull natural gems are absent, yet they betray their origin in other ways. Flame-fusion gems may hold small trapped air bubbles, visible banding, or crack-like chatter marks from polishing, the quiet fingerprints that tell a gemologist where a crystal was born.