How to Tell a Lab-Grown Diamond from a Natural One

A lab-grown diamond and a natural diamond of the same colour, clarity, cut, and carat have identical visual properties¹. The refractive index is 2.42 in both cases, which gives diamonds their characteristic ability to bend and reflect light. The dispersion (the splitting of white light into spectral colours, what gemmologists call fire) is 0.044 in both. The hardness is 10 on the Mohs scale, the highest of any natural mineral, in both. Density is approximately 3.52 g/cm3 in both.

A trained gemmologist examining a stone under 10-power magnification can sometimes spot characteristic features. HPHT-grown stones often contain small metallic flux inclusions (dark specks). CVD-grown stones may show layered growth lines under the right lighting. But these are not reliable across the full population of lab-grown stones, and high-quality lab-grown rough may show no obvious gemmological tell at all.

The handheld diamond tester used in most jewellery stores is a thermal-conductivity instrument. Diamond conducts heat exceptionally well (about five times better than copper), which is what the tester measures. Cubic zirconia, glass, and most other common simulants conduct heat much less effectively, and the tester reads them as not-diamond.

Lab-grown and natural diamonds conduct heat the same way, because they are the same material. The tester reads both as diamond, correctly. It does not and cannot distinguish between them. A separate moissanite tester (silicon carbide has unusually high thermal conductivity that can fool a standard diamond tester into a false positive) is similarly ineffective for lab-grown vs natural identification, because it tests for a different property entirely.

The practical implication is that any retailer or appraiser who uses only a thermal-conductivity tester is not testing for laboratory origin. They are testing for diamond at all. Identification requires laboratory equipment.

Gemmological laboratories use a combination of spectroscopic and imaging methods to distinguish lab-grown from natural diamonds¹³.

Photoluminescence spectroscopy

Photoluminescence (PL) spectroscopy measures the emission of light from a stone after excitation by a laser, typically at liquid-nitrogen temperatures. The emission patterns are sensitive to the defect structure of the diamond, which differs by growth history. Natural diamonds typically show characteristic PL features at wavelengths associated with nitrogen aggregation (the H3, H4, and 595 nm centres) that develop over geological time. CVD-grown diamonds typically show silicon and nitrogen-vacancy features (the SiV at 737 nm). HPHT-grown diamonds show different signatures. PL is one of the most diagnostic single tests for laboratory origin.

Cathodoluminescence imaging

Cathodoluminescence (CL) imaging illuminates the stone with an electron beam and records the visible-light response. The image reveals internal growth structure: layered horizontal lines for CVD, sector zoning for HPHT, complex growth horizons and post-growth dislocations for natural³. The visual fingerprint of growth method is unmistakable in CL imaging, although it requires laboratory equipment and prepared samples.

DiamondView ultraviolet imaging

The DiamondView instrument, developed by De Beers' research arm and widely used in commercial gemmological laboratories, illuminates the stone with shortwave ultraviolet light and records the fluorescence pattern³. The fluorescence pattern is a non-destructive analogue of CL imaging. CVD stones show characteristic layered fluorescence. HPHT stones show sector-zoned fluorescence. Natural stones show different patterns again. DiamondView is the workhorse of in-laboratory lab-grown screening.

GIA iD100 spectroscopic screening

The GIA iD100 is a more compact spectroscopic instrument designed for screening of mounted and loose stones outside of full laboratory analysis². It is used in trade and at some retail points to flag stones that require further laboratory analysis, rather than as a final identification tool. The iD100 distinguishes natural type Ia diamonds (the dominant natural type) from CVD-grown and HPHT-grown stones with high reliability for the bulk of submitted material, and refers to laboratory PL or DiamondView for the harder cases.

Most lab-grown diamonds carry a laser inscription on the girdle, the thin polished band around the stone's widest circumference¹. The inscription typically begins with LG (for laboratory-grown), followed by the laboratory's certificate number. The inscription is invisible to the naked eye but readable under 10-power magnification with the right viewing angle.

The inscription is not strictly mandated by US federal law for all lab-grown stones, but it is industry standard practice for IGI and GIA-graded lab-grown diamonds and is required or strongly recommended in several jurisdictions. The inscription serves three purposes: it allows a buyer to match the stone to its grading report by matching the inscribed number against the certificate, it provides a permanent visual flag of laboratory origin, and it provides an authenticity check (the inscription should validate against the laboratory's online database).

Outside a gemmological laboratory, the practical verification steps a consumer can take are limited but useful⁴:

1. Read the certificate. The grading report from GIA or IGI explicitly states whether the stone is laboratory-grown or natural, what method (HPHT or CVD) was used for lab-grown, and any post-growth treatments.
2. Match the laser inscription. The number on the certificate should match the inscription on the girdle, viewed under magnification.
3. Verify the report online. Both GIA and IGI offer online certificate verification by report number. The laboratory's database should return the same details that appear on the physical certificate.

What a consumer cannot do alone is distinguish lab-grown from natural without paperwork. There is no home test for laboratory origin. If a stone arrives without a credible certificate from a recognised laboratory, the only reliable identification path is submission to a gemmological laboratory for analysis.

Where this fits in the reference

Identification technology and the FTC disclosure rules (Chapter 6) work together: rules require disclosure at point of sale, instruments verify the disclosure when needed. The next and final chapter, Resale and Value, addresses what happens when a buyer wants to sell a stone back to the secondary market.