Synthetics & simulants

Diamond is grown by two routes. High-pressure, high-temperature (HPHT) growth imitates conditions in the mantle. Carbon and a small diamond seed sit in a molten metal solvent, usually iron, nickel or cobalt, inside a belt, cubic or split-sphere press. Gem growth runs at about 5–6 GPa and 1,300–1,600 °C. At General Electric, Tracy Hall reached a reproducible synthesis on December 16, 1954, using a belt press with graphite and an iron sulfide catalyst at roughly 7 GPa and 1,600 °C, and the team published in Nature in 1955. Researchers at the Swedish company ASEA had made diamond in 1953 but did not report it at the time, and credit for the first synthesis is still argued over. Herbert Strong and Robert Wentorf of General Electric went on to grow gem-quality crystals up to 6 mm across over several days.

Chemical vapor deposition (CVD) builds diamond from gas. Hydrogen with a small fraction of a hydrocarbon, generally under 10% methane, is energized into a plasma at roughly 20–500 mbar, and carbon settles layer by layer on seed plates held at 600–1,200 °C. CVD crystals often grow brownish and are frequently given post-growth HPHT treatment.

Both routes yield real diamond. Since July 2019 GIA has issued a Laboratory-Grown Diamond Report that names the growth method.

Four methods produce nearly all synthetic colored stones. Flame fusion, announced by the French chemist Auguste Verneuil in 1902, drops powdered aluminum oxide mixed with a coloring agent through an oxygen-hydrogen flame onto a rotating support, where it builds a cylindrical crystal called a boule. It is the cheapest method and still accounts for most laboratory-grown ruby and sapphire. Verneuil stones show curved striae and spherical gas bubbles, neither of which occurs in natural corundum.

Czochralski pulling, developed by Jan Czochralski in 1918, draws a crystal upward from a melt on a rotating seed and yields large clean boules of synthetic sapphire, alexandrite and yttrium aluminum garnet (YAG).

Flux growth dissolves the ingredients in a molten solvent such as lithium molybdate, which deposits crystals slowly at lower temperatures than melt methods. Richard Nacken grew the first facetable synthetic emeralds in the mid-1920s, IG Farben sold Igmerald from 1935, Carroll Chatham produced emerald from 1941, and Pierre Gilson's French factory held about 95% of world output by the mid-1970s. Flux stones carry platelets of platinum from the crucible and flux-filled fingerprints.

Hydrothermal growth crystallizes gems from hot pressurized water in sealed autoclaves. Johann Lechleitner applied it to emerald around 1960, and Union Carbide's Linde division disclosed its own process in patents soon after. Hydrothermal emerald shows chevron graining and nailhead spicules.

A simulant copies appearance, not identity. Cubic zirconia (CZ), zirconium oxide stabilized with yttrium or calcium oxide, is the most common diamond imitation. Researchers at the Lebedev Physical Institute in Moscow developed the skull-melting method that made it practical and published on it from 1973; commercial production followed in 1976. CZ has a refractive index of 2.15–2.18 against diamond's 2.42, stronger dispersion (the splitting of white light into spectral colors) at 0.058–0.066, and a Mohs hardness of 8–8.5. Its specific gravity of 5.6–6.0 against diamond's 3.52 means a CZ weighs considerably more than a diamond of the same size.

Synthetic moissanite, silicon carbide, reached the jewelry market in the late 1990s and is the closest imitation. Its refractive indices are 2.648 and 2.691, its dispersion 0.104 and its hardness 9¼. Its thermal properties are close enough to diamond that thermal probes react to it as if it were diamond. It is doubly refractive, so facet junctions look doubled under magnification.

Older simulants survive in vintage jewelry. Synthetic rutile reached the market in 1948, strontium titanate in 1955, YAG in 1968 and gadolinium gallium garnet (GGG) in 1975. GIA also lists glass and colorless zircon among the materials sold to stand in for diamond.

A composite, or assembled stone, is two or more pieces of material glued or fused together in the form of a single gem. GIA defines a doublet as two joined segments and a triplet as three segments, or as two segments separated by a layer of colored cement.

Assembled stones are not always imitations. Precious opal often forms in layers too thin to set, so an opal doublet bonds a slice to a dark backing of onyx, plastic or natural matrix, and an opal triplet adds a clear dome of rock crystal, glass, plastic or synthetic corundum that protects the thin opal and magnifies its color. Other types are meant to deceive. Garnet-topped doublets fuse a thin slice of red garnet to a colored glass base, giving the crown the harder, more lustrous surface and the inclusions of a natural stone. Sapphire doublets pair a natural crown with a synthetic pavilion.

Composites are exposed at the join. A gemologist looks along the girdle and from the side for a separation plane, for bubbles trapped in the cement and for a change in luster between crown and pavilion. The US Federal Trade Commission requires that a stone made of gem material bonded with a filler such as lead glass not be sold under a plain gem name.

Standard gemology still identifies most synthetic colored stones by their inclusions: curved striae and gas bubbles in flame-fusion corundum, platinum platelets and flux fingerprints in flux-grown stones, chevron graining in hydrothermal emerald. Laboratory-grown diamonds are harder, because they are diamond.

Laboratories rely on luminescence imaging and spectroscopy. DiamondView, built by the De Beers diamond research group, floods a stone with ultraviolet light below 225 nm and photographs the fluorescence, which maps growth structure. HPHT crystals show a cuboctahedral arrangement of growth sectors; CVD crystals show fine parallel growth layers. Under crossed polarizers an absence of strain is a strong sign of HPHT growth, while CVD diamonds show a characteristic strain pattern. Photoluminescence spectroscopy, done at liquid-nitrogen temperature, records emission from trace defects: a silicon-vacancy doublet at 736.6 and 736.9 nm marks CVD growth, and nickel-related peaks point to HPHT growth. Newer CVD material contains less silicon, so that marker is weakening.

Small stones, called melee, are screened in bulk. GIA's iD100 tests loose or mounted colorless to near-colorless diamonds of 0.9 mm (about 0.005 ct) and larger in under two seconds and returns “pass” or “refer.” GIA says no rapid, low-cost screening tool identifies every laboratory-grown diamond, so referred stones go to a laboratory. GIA inscribes “Laboratory-Grown” on the girdle of every lab-grown diamond it reports on.

Lab-grown diamond prices fell steeply as producers learned to grow stones faster and in larger volume. De Beers, announcing on May 8, 2025 that it would close its Lightbox jewelry brand, said wholesale prices for lab-grown diamonds in jewelry had fallen 90% and were tracking closer to a cost-plus model. Its own Diamond Report puts the fall at 93% since 2020 and average lab-grown wholesale prices at about US$100 per carat. Lightbox had launched in 2018 at a flat US$800 per carat.

Cheap stones sold in volume without taking the value. De Beers research published in June 2026 found that lab-grown stones made up 15% of independent US jewelers' diamond sales in 2025, against 85% for natural diamonds, and that falling retail prices held their share of sales value down. Average spending on a piece of natural diamond jewelry rose to US$4,063 in 2025 from US$3,242 in 2023.

Rough production fell over the same period. Kimberley Process figures put world rough output at 98.8 million carats in 2025, worth US$9.23 billion, down from 107.9 million carats in 2024.

The US Federal Trade Commission's Jewelry Guides define a diamond as a mineral consisting essentially of pure carbon crystallized in the isometric system, a definition that covers lab-grown stones. A seller must qualify the word with equal conspicuousness, using “laboratory-grown,” “laboratory-created,” “[manufacturer name]-created” or similar wording. “Cultured” is allowed only alongside such a qualifier. Imitations must be called “imitation” or “simulated,” and “faux” is not adequate disclosure. The Guides were last amended in 2018.

The World Jewellery Confederation (CIBJO) is stricter. Its Diamond Blue Book permits only “synthetic,” “laboratory-created” or “laboratory-grown,” treats the three as synonyms that may not be abbreviated, and forbids “cultured,” “cultivated,” “real,” “genuine,” “natural,” “gem” and “gemstone.” On September 7, 2026 the CIBJO board went further, recommending “synthetic” as the sole descriptor for international use and treating the other two as marketing terms accepted in some countries.

Laboratories have changed their language as well. GIA replaced its Synthetic Diamond Grading Report with the Laboratory-Grown Diamond Report on July 1, 2019. On June 2, 2025 it announced that it would stop applying its natural color and clarity scales to lab-grown diamonds, on the ground that more than 95% of them fall in a narrow range, and describe them instead as “premium” or “standard.”