A laboratory-grown diamond is not a simulant. It is diamond — crystalline carbon, hardness 10, refractive index 2.417, the same thermal conductivity, the same sparkle. Every test that separates diamond from moissanite and cubic zirconia says "diamond" for a grown stone, because it is one. The question of whether a diamond grew in the earth or in a reactor is answered by a different set of methods, and it matters because the two are priced very differently.
The two growth methods
HPHT (high pressure, high temperature). Carbon is dissolved in a molten metal catalyst (iron, nickel, cobalt) at pressures of around 5–6 GPa and temperatures around 1,300–1,600 °C — conditions that reproduce the diamond-stable region of the earth's mantle — and crystallises on a seed. The process has been used industrially since the 1950s and produces gem-quality stones since the 1980s–90s. HPHT is also used to treat natural and CVD diamonds, improving their colour.
CVD (chemical vapour deposition). A diamond seed plate is placed in a vacuum chamber with a hydrocarbon gas (typically methane and hydrogen) that is energised into a plasma at moderate pressure and around 800–1,200 °C. Carbon atoms deposit on the seed layer by layer, growing a tabular crystal upward. CVD has become the dominant method for gem diamond since the 2010s. Many CVD stones are HPHT-treated afterwards to remove a brownish tint.
Why the standard tests fail
Thermal conductivity, electrical conductivity (with exceptions), refractive index, specific gravity, hardness and dispersion are properties of diamond as a material and are identical in natural and grown stones. The thermal tester beeps for both. The refractometer reads over the limit for both. The loupe finds no doubling in either. Only features that depend on how the crystal grew can separate them.
What does separate them
Growth-related features. HPHT diamonds grow with cubic and octahedral sectors that can show under UV as a cross-shaped or geometric fluorescence pattern, and may contain tiny metallic inclusions from the catalyst — sometimes magnetic. CVD diamonds grow in layers on a flat seed and can show striations, a characteristic strain pattern under crossed polarisers (fine banding rather than the "tatami" mottle of natural diamond), and occasionally dark pinpoint carbon inclusions.
Fluorescence and phosphorescence. Grown diamonds often show unusual fluorescence colours and patterns under short-wave UV and, in many HPHT stones, persistent phosphorescence after the lamp is switched off. Instruments built on this principle image the stone's UV response and are used as screening tools.
Type classification. Most natural diamond is Type Ia, containing aggregated nitrogen. Most grown diamond is Type IIa (near-nitrogen-free) or Type IIb (boron-bearing). Type II is rare in nature — a few percent — so a Type II stone is referred for further testing, not condemned. Screening devices work on this basis: they pass Type Ia stones as natural and refer Type II stones.
Spectroscopy. Photoluminescence (PL) spectroscopy at liquid-nitrogen temperature, FTIR and UV-Vis detect specific defects and features — silicon-vacancy centres in CVD, nickel-related centres in HPHT, and the absence of the natural nitrogen-aggregation history — that constitute the confirmation. This is the level at which the determination is actually made.
Inscriptions. Many grown diamonds are laser-inscribed on the girdle with "LG", "Lab Grown" or the grower's mark. The inscription can be polished off; its absence proves nothing.
What a routine laboratory can do
A bench laboratory can screen: check for inscriptions, examine UV fluorescence and phosphorescence, look for growth patterns under crossed polarisers and for metallic inclusions, and run a type-screening device if it has one. It can pass a stone as showing no indications of laboratory growth, or refer it. It cannot, without PL and FTIR spectroscopy, issue a definitive origin determination on a referred stone. A report should be honest about which of these was done. What a laboratory report cannot tell you.
The disclosure question
Grown diamonds are a legitimate product and are sold openly at prices well below mined stones. The problem is undisclosed substitution — in loose stones, in set jewellery, and increasingly in melee (small stones) where individual testing is impractical. A buyer paying a natural-diamond price is entitled to a report from a laboratory equipped to make the determination.
Frequently asked questions
Is a lab-grown diamond a real diamond?
Physically and chemically, yes. It differs from a mined diamond in origin and in price, not in material.
Can a jeweller's diamond tester detect lab-grown?
No. It tests for diamond, and a grown diamond is diamond.
Does "Type IIa" mean lab-grown?
It means the stone is nitrogen-poor, which most grown diamonds are and few natural ones are. It is a reason for further testing, not a verdict — fine natural Type IIa diamonds exist.
Can lab-grown diamonds be treated?
Yes. Many CVD stones are HPHT-treated to improve colour. Treatment and origin are reported separately.
How can I be sure a diamond is natural?
A report from a laboratory with spectroscopic capability, with weight and dimensions that match the stone, and — for set jewellery — ideally an inscription cross-referenced to that report.