How to identify a diamond

Let us begin by dismantling what you have probably already read elsewhere.

The case of the thermal tester deserves a word, because inexpensive ones can be found almost everywhere. It measures thermal conductivity, which is very high in diamonds — but moissanite also conducts heat well. A thermal tester alone will therefore identify it as a diamond. A combined thermal and electrical device is needed to distinguish between the two.

With a ten-power loupe — the gemmologist’s basic tool, which costs a few dozen euros — two observations are genuinely informative.

Facet doubling. This is the most effective test for moissanite, the leading imitation today. Moissanite is birefringent : it splits the rays of light that pass through it. Looking through the table at a slight angle, one can see the edges of the lower facets appear doubled, like a ghost image. A diamond never produces this effect.

The condition of the edges. Diamond is the hardest natural material: its edges remain sharp and crisp even after decades of wear. Cubic zirconia, which is much softer, becomes worn and chipped along the girdle and corners. On an antique ring, this is often the first clue.

Let us distinguish between two categories that are often confused. An imitation is another material that resembles a diamond. A synthetic diamond is a genuine diamond, produced in a laboratory — we will come to that shortly.

A word about white sapphire, since it is our trade: it is not a counterfeit. It is a colourless corundum, a genuine gemstone, which we readily use as an accompanying stone. It simply does not have the dispersion of a diamond — that ability to break light down into coloured flashes. Placed beside a diamond, it appears softer and milkier. Sold as a diamond, however, it is deceptive.

Here is what no home-testing article states clearly: no at-home method can distinguish a laboratory diamond from a natural diamond. None. Not condensation, not a tester, not a loupe.

The reason is simple: a laboratory diamond is a diamond. Same composition, same crystalline structure, same hardness, same dispersion. It is not fake — it simply has no geological origin.

Only an analysis in a gemmological laboratory can determine this, by observing the characteristic inclusions and growth structures, sometimes using spectroscopic equipment. This is beyond the reach of a private individual, and even of a jeweller without specialised equipment.

The reliable answer lies in a document and a number. An independent laboratory report — GIA, IGI, HRD for diamonds — describes the stone, its dimensions, colour, clarity and cut, and states whether it is natural or laboratory-grown.

Two checks anyone can carry out. First, the report number can be verified online on the laboratory’s website: if the report exists, its details will appear. Secondly, many certified diamonds bear a laser inscription on the girdle — the edge of the stone —, invisible to the naked eye but legible under a loupe, displaying the report number.

This is the most direct link between a document and a gemstone. Without it, a certificate proves thata stone has been analysed, not that it is this one. Checking the dimensions with callipers usefully completes the verification.

In practice, a thorough identification takes a few minutes and bears no resemblance to any at-home test.

The stone is first observed with the naked eye from twenty centimetres away, then under a 10x loupe, looking for facet doubling, the nature of inclusions and the condition of the edges. The stone is then compared with reference gems under the same light — often the most decisive step. Finally, it is measured with callipers and weighed, to compare its dimensions with the expected density.

If any doubt remains, or if the stakes warrant it, the stone is sent to a laboratory. This is the only way to determine whether it is of natural or synthetic origin, and no honest jeweller will claim otherwise.

One final observation, which goes beyond the question of authenticity. Many people seek to identify a diamond because they assume that a serious ring contains one. This is a recent and largely constructed idea: the diamond only became established in engagement rings in the 20th^th century. Before it, coloured stones held the field.

Hardness 9

Sapphire

Ten colours, no cleavage planes, and easy identification for a gemmologist. Set ring, 2.71 ct.

11 040 €

 

Hardness 7.5–8

Emerald

Its garden of inclusions is its signature: this is precisely what makes it easy to identify as natural. Set ring, 1.34 ct.

8 400 €

 

Loose stone · hardness 9

Ruby

The same mineral as sapphire, and the rarest of the major gemstones above two carats. Mozambique, 1.53 ct.

30 060 €

 

Availability and prices including VAT observed in September 2026.

Coloured gemstones also have an advantage in this particular respect: their inclusions are their identity card. Where a laboratory-grown diamond perfectly imitates a natural diamond, synthetic corundums and beryls leave signatures that gemmology can read. A ruby engagement ring with certification raises fewer questions of authenticity today than a white diamond without papers.

This does not mean giving up diamonds — we set them in almost all our creations, as marquise cuts, baguettes and pavé. Simply that the question “is it real?” arises less acutely when one moves from white to colour.