Definitions & classification

Gemologists judge a candidate gem on three qualities: beauty, rarity and durability. CIBJO’s Gemstone Book names the same three in its definition of a gemstone. Beauty covers color, transparency, luster (the way a surface reflects light) and optical effects such as the play-of-color in opal. Rarity matters because a material found by the ton, however attractive, seldom commands a high price. GIA splits durability into hardness (resistance to scratching), toughness (resistance to chipping and breaking) and stability (resistance to chemicals, heat, humidity and light).

Few stones score well on all three. Diamond is the hardest material in nature, 10 on the Mohs scale, yet a sharp blow along certain planes can still split it. Pearl, at 2.5–3 on the same scale, and opal, at 5 to 6.5, are soft by comparison, which is why GIA suggests keeping soft stones for occasional wear. Some minerals are cut only for collectors because they would scratch or break in a ring.

The definition is cultural as well as physical. A material becomes a gem when people choose to wear, carve or trade it, and fashion, new deposits and treatments shift that choice over time.

Western tradition long divided gems into two tiers. Diamond, ruby, sapphire and emerald were precious; everything else was semi-precious. The split reflected rarity, hardness and prestige as earlier centuries saw them, not any scientific property.

The labels have aged badly. GIA records that amethyst was as expensive as ruby and emerald until the 19th century, when Brazil’s large deposits were found. A fine green tsavorite garnet, nominally semi-precious, can be worth far more than a mid-quality emerald, while low-grade material of a “precious” species can be inexpensive. Price depends on the individual stone rather than its category.

The trade bodies have moved on. CIBJO’s Gemstone Book calls the term “semi-precious” misleading and says it shall not be used, and allows “precious” only for natural materials. The US Federal Trade Commission’s Jewelry Guides make it deceptive to describe an artificially produced product as “real”, “genuine”, “natural”, “precious” or “semi-precious”. Retailers still use “precious” as shorthand for the four traditional stones, and many dealers group everything except diamond as colored stones.

The Big Four are diamond, ruby, sapphire and emerald, the stones that traditionally carried the “precious” label. They combine strong demand, long use in royal and religious jewelry, and enough hardness for daily wear: 7.5–8 on the Mohs scale for emerald, 9 for ruby and sapphire, and 10 for diamond.

Mineralogically the group is smaller than it sounds. Ruby and sapphire are both corundum, aluminum oxide, and differ only in the trace elements that color them. Chromium makes ruby red, while iron and titanium make sapphire blue. Emerald is a green variety of beryl, the same species that produces aquamarine. Diamond stands apart as the only gem made of a single element, carbon.

The four also have the most developed trade infrastructure. Diamond has its own grading scales, price lists and bourses, largely separate from the colored-stone trade, and for ruby, sapphire and emerald a laboratory opinion on treatment and geographic origin can change the price substantially. Those services recur throughout the later chapters on grading, laboratories and markets.

Most gems are minerals. The International Mineralogical Association defines a mineral as a homogeneous solid formed by geological processes, with a definite chemical composition and an ordered atomic arrangement. Diamond, corundum, beryl and quartz all qualify. Substances built entirely inside a living organism, pearl among them, do not.

Several important gems do not. A rock is an aggregate of one or more minerals: lapis lazuli is a rock made mostly of blue lazurite with white calcite and brassy pyrite. A mineraloid is natural but lacks an ordered structure, and opal and obsidian are the usual examples. Organic gems come from living things. Pearl grows inside mollusks, precious coral is an animal skeleton, amber is fossilized tree resin and jet is a type of lignite formed from ancient wood.

Laboratory products form another family. A synthetic or laboratory-grown stone has essentially the same chemistry, crystal structure and properties as its natural counterpart, as with lab-grown diamond. A simulant, or imitation, only resembles the gem it copies: cubic zirconia imitates diamond without sharing its composition. Composite stones such as doublets join two or more pieces. Under the US Federal Trade Commission’s guides, sellers must qualify such products with words like “laboratory-created” or “imitation.”

Gemology borrows its core hierarchy from mineralogy. A species is defined by its chemical composition and crystal structure. A variety is a named subdivision of a species, usually based on color or an optical effect, and it has no separate chemical formula.

Corundum shows how this works. Every ruby and every sapphire is crystalline aluminum oxide, Al₂O₃, with the same hardness and nearly the same density and refractive index. Traces of chromium produce red, which the trade calls ruby. Iron and titanium produce blue sapphire, and other traces give the yellow, pink, orange and violet stones sold as fancy sapphires. Beryl follows the same pattern: chromium or vanadium makes emerald, ferrous iron makes aquamarine and manganese makes morganite.

Above species sits the group, a family of species that share a structure but differ in chemistry. Garnet and tourmaline are groups, which is why “garnet” covers stones as different as red pyrope and green demantoid.

Variety boundaries rest on trade convention rather than chemistry, so they can be disputed. GIA, for example, uses graded comparison stones to decide whether a green beryl is saturated enough to be called emerald. Such calls matter because the variety name often sets the price.

Every reference table in this encyclopedia, and every laboratory report, describes a stone with the same short list of measurements. Crystal system is the symmetry of the atomic lattice. Crystallography recognises seven systems, from cubic (diamond, spinel, garnet) to trigonal (corundum, quartz), and the system decides how a crystal grows and how it behaves in polarised light. Mohs hardness is a scratch order, not a scale of equal steps. Refractive index records how much the stone slows and bends light; gemologists read it on a refractometer, and it is the single most useful identification figure for a cut stone. Specific gravity is the ratio of the stone's density to the density of water, so an SG of 3.52 means the stone weighs 3.52 times what the same volume of water weighs. Gemologists weigh a stone in air and again suspended in water to work it out.

Two stones can share a color and part with a single figure: quartz reads 1.54 and diamond 2.42. Chapter 3 goes into the physics behind each measurement, and chapter 12 into the instruments that take them.

Gems are weighed in carats. The metric carat is exactly 200 milligrams, one-fifth of a gram, and CIBJO’s Gemstone Book requires the weight to be stated to two decimal places. Before it spread, carat weights differed from one trading city to the next: the US Bureau of Standards recognised the 200 mg carat from 1 July 1913, by which time most European trading countries had already adopted it. One carat divides into 100 points, so a 0.25 ct diamond is a “25-pointer.” US Federal Trade Commission guidance says a decimal weight should be accurate to its last stated place, so a stone sold as “.5 carat” should weigh between 0.495 and 0.504 ct.

Dealers quote prices per carat, and the total price is weight multiplied by that rate. The rate itself climbs with size because large stones are scarcer, so a 2 ct stone costs more than twice as much as a comparable 1 ct stone. Price lists group stones into weight brackets, and the per-carat rate steps up at the start of each bracket.

The steps are steepest at magic sizes. GIA names 1.00, 1.50 and 2.00 carats: a 0.99 ct diamond can carry a noticeably lower per-carat price than a comparable 1.01 ct stone, although the two look nearly identical once set. Cutters therefore have an incentive to finish stones just above these thresholds.

Rough is uncut gem material as it leaves the mine or sorting house; polished goods have been cut and are ready for grading or setting. Cutting always costs weight. A Gems & Gemology study of Indian factories working small goods reported yields of roughly 15–25% of the rough weight, one reason rough and polished prices do not move in step.

Melee are small diamonds, often given simplified cuts with fewer facets and set in clusters, bands and pavé. GIA defines melee as small diamonds, single or full cut, under a fifth of a carat, running down to about 0.001 ct; where the trade draws the line varies by market. Melee and small colored stones are often calibrated, cut to standard millimeter dimensions so that a manufacturer can set them in ready-made mountings without refitting each one.

Small stones rarely travel alone. They are sold in parcels, lots sorted by size, color or quality and priced per carat for the whole lot. Larger goods often move on memo, short for memorandum: a consignment arrangement in which a supplier lends stones to a retailer or another dealer, who may sell them or return them within an agreed period while ownership stays with the supplier. Memo lets a retailer show stock it has not paid for, but it exposes the supplier to the borrower’s credit and security risks.