HPHT laboratory-grown diamonds that test as “synthetic moissanite”

By Julia Griffith FGA DGA EG

Originally published on Retail Jeweller online on 15th March 2021

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Diamond testers can be a handy tool for separating diamonds from imitations. However, millions of laboratory-grown diamonds (LGDs) in the jewellery market give false results. Those that rely on diamond testers could misidentify these stones.

LGD’s and natural diamond are the same material and, therefore, share the same properties. One would expect them to react exactly the same to all diamond testers. However, this is not always the case.

Diamond multi-testers assess the thermal and electrical conductivity of materials. Thermal conductance separates the majority of imitations from diamond except for synthetic moissanite. These stones are separated from diamonds via electrical conductance, as synthetic moissanite conducts electricity, whereas diamonds do not.

A 0.59ct HPHT laboratory-grown diamond, which has been graded a G-colour by IGI and has a subtle blue tint due to boron content.

There is one exception to this rule: diamonds containing boron as minor impurities. Natural diamonds containing boron are exceptionally rare, accounting for less than 0.01%. These diamonds are coloured blue and are highly coveted.

Some LGD’s grown by high-pressure, high-temperature (HPHT) synthesis conduct electricity for the same reason – boron. The impurity levels are so low that they typically show just a subtle tint of blue. These stones appear near-colourless and are sold on the commercial diamond market as colourless LGD’s.

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A "E" colour diamond (left), next to a HPHT laboratory-grown diamond with a blue nuance.

The subtle blue tint is known as a “blue nuance”. It is estimated that roughly 70% of near-colourless HPHT laboratory-grown diamonds, equivalent to colour grades G and below, are semi-conductors of electricity.

All users of diamond testers need to be aware of this false result as the incorrect identification of laboratory-grown diamonds could hurt consumer confidence and professional reputation.

Pawnbrokers, valuers and second-hand retailers who rely on diamond testers to identify diamonds versus imitations must perform further observations on stones that test as “synthetic moissanite” to ensure that this is the correct identity.

A laboratory-grown diamond report by IGI stating "Blue nuance" for a G-colour HPHT laboratory-grown diamond.

A key observation that will determine the stone's identity is evidence of optical nature. “Doubling” indicates a strongly doubly refractive material. This observation, paired with electrical conductivity, confirms the stone is synthetic moissanite. A lack of “doubling” in an electrically conductive stone indicates diamond and will most likely be one of these HPHT lab-grown stones.

It is important to note that subtle blue tints can occur in natural diamonds. Again, these are very rare and highly prized. These stones have usually been authenticated as natural by a grading laboratory and circulate the coloured diamond market. However, due to the slight possibility that a natural light blue diamond could enter the colourless diamond market - it is advisable to obtain additional proof of origin when a lab-grown diamond is suspected. This could be a characteristic observation, such as inclusions or laser engravings. Basic tests are also helpful, such as checking for fluorescence and phosphorescence with UV light and looking at strain patterns under crossed polarised filters.

Listen to The Gem Show Podcast which discusses this in more detail.

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