Lab-Grown Diamond

Grown, not found · Field guide

Direct twin

C  ·  carbon  ·  cubic

Sold as: lab-grown diamond, laboratory-created diamond, HPHT diamond, CVD diamond, and 'cultured diamond', which US FTC guides allow only with a clear qualifier beside it.

Two completely different manufacturing routes, two different sets of tell-tales, and one material that is chemically and optically indistinguishable from the mined version without instruments.

Mohs

10

Refractive index

2.417

Birefringence

none

Dispersion

0.044

Specific gravity

3.52

Crystal system

Cubic

Cleavage

Perfect octahedralfour directions

Thermal conductivity

Extreme

HPHT

1. 1Carbon source at the hot end of the capsule
2. 2Molten metal solvent (iron, nickel, cobalt) that carbon dissolves into
3. 3Diamond seed at the cooler end. Carbon crosses the gradient and crystallizes on it

The original route, and still the cheaper one for colored and smaller material. A capsule holding a carbon source, a molten metal solvent — usually iron, nickel or cobalt — and a diamond seed is squeezed to 5–6 GPa at 1300–1600 °C. Carbon dissolves in the metal at the hot end, migrates down the temperature gradient, and crystallizes on the seed at the cool end.

HPHT crystals grow as cuboctahedra, with a mixture of cube and octahedron faces that natural diamond rarely produces — natural crystals are octahedral or dodecahedral. They usually contain traces of the metal solvent, which makes many of them faintly magnetic, something no natural diamond is. Under specialist fluorescence imaging they show a distinctive cross or hourglass growth pattern.

Without a nitrogen getter in the capsule, HPHT diamond comes out yellow (type Ib). Most natural diamond is type Ia, with nitrogen aggregated into pairs and clusters over geological time. That normally takes millions of years, though HPHT treatment can force some of it in a press, so type alone isn't proof.

CVD

1. 1Hydrogen and a trace of methane at low pressure
2. 2Microwave-driven plasma cracks the gas into reactive carbon and atomic hydrogen
3. 3Diamond seed plates. The crystal builds up in layers, roughly 10–30 µm per hour

A low-pressure atmosphere of hydrogen with a percent or two of methane, energized into a plasma by microwaves. The plasma cracks the gas; carbon deposits on diamond seed plates while atomic hydrogen etches away anything that forms as graphite instead. Growth runs at maybe 10–30 microns per hour, so a gem-sized crystal takes weeks.

CVD diamond grows as a flat tablet — nothing like a natural crystal shape — with parallel striations on the growth surface. As-grown material is often brownish, and is routinely HPHT-annealed afterwards to remove that, which means many CVD stones have been through both processes. Many carry a silicon-vacancy defect that produces a diagnostic emission line at 737 nm, a signature natural diamond almost never shows.

Telling them apart

Why it's here

Diamond isn't lapidary rough for most people — it needs specialized equipment and a different set of skills. It's on this site because it is the clearest illustration of the site's three questions: an abundant natural twin, two distinct growth routes that leave different fingerprints, and an origin story that starts with abrasive grit rather than jewelry.

A reference to lab-grown gem materials, written for cutters. Values are typical published figures; ranges vary with composition, dopant and measurement wavelength. Nothing here is a substitute for a laboratory report on a stone whose identity matters commercially.