THE AUSTRALIAN GEMMOLOGIST | Surface-Coated Synthetic Moissanite Now Widely Available
Surface-Coated Synthetic Moissanite Now Widely Available
Introduction
The Victorian Division of the GAA has just purchased a parcel of interesting, surface-coated moissanites to add to the gemmological collection. These brightly coloured stones come with certificates and are laser inscribed on the girdle. While the process of identifying the treatment is not complex, it is important that gemmologists are aware of the existence of these stones on the market.
The stones are available in over twenty colours, including a range of ‘mystic’ iridescent colours (Figure 1), and can be found online from multiple websites. They are being supplied with suspicious-looking certificates branded ‘GRA’ that could be mistaken for ‘GIA’ certificates at first glance.
Moissanite is silicon carbide (SiC) and crystallises in the hexagonal crystal system. Commercially, synthetic moissanite has been available since 1998 as a diamond imitant, initially manufactured and sold by Charles & Colvard who owned the patents.
Moissanite is an excellent imitant of diamond due to its very high refractive index, hardness, lustre and dispersion (Table 1). However, due to moissanite crystallising in the hexagonal crystal system, it is doubly refractive which is a key separation point from diamond. Moissanite’s very high birefringence and dispersion are also key points of difference, with the stone usually cut with the table perpendicular to the optic axis to mask the doubling.
All patent rights for synthetic moissanite production expired in 2018. Since then, there has been a significant increase in production and availability of moissanite, together with a marked decrease in the price of the product.
Coloured moissanite has been widely available for many years but has been limited to tones of brown, green, light blue, teal and black. Brighter colours of blue and yellow, together with pinks and reds, have not been seen on the market until recently with the application of surface coatings.
Figure 1. A sample of colours of commercially available synthetic moissanite.
Table 1. Physical and optical properties of diamond and synthetic
moissanite.
Identification of Moissanite and Surface Coating
An important feature in the identification process for synthetic moissanite has always been to recognise the high birefringence. Doubling of the back facet edges is usually seen when looking through a crown facet and looking towards the culet. Remember that due to the stones being cut with the table perpendicular to the optic axis, no doubling of back facets will be observed directly through the table. However, there is one tell-tale feature that is visible through the table that will always be doubled and will serve as a quick visual separation from diamond. The table reflection in the pavilion of a diamond is used by diamond graders to estimate the depth of the pavilion of the stone. Because diamond is singly refractive, only one table reflection is seen. But this table reflection is also immediately recognisable in moissanite as it is ‘doubled’, and this provides a quick confirmation of the high birefringence of the stone (Figure 2).
Identifying the surface coating on these coloured moissanites is straightforward. Nevertheless, as coating on moissanite has been encountered infrequently, it is unlikely that most gemmologists would be looking for evidence of this treatment and it is easy to miss. These stones have been coated on the pavilion only, so tell-tale signs of coloured metallic lustre are not visible on the crown. This will make identification of this treatment more difficult for stones that are set in jewellery.
All the stones display metallic lustre on the pavilion, with superficial surface damage seen in the coating. Due to moissanite’s hardness of 9.25, surface scratches are not commonly seen, but all stones studied here showed fine scratches to the surface coating (Figure 3) and minor loss of coating to the facet edges; when the damage was more prominent, a loss of colour could be observed with the handlens.
Depending on the colour of these stones, they featured either an iridescent metallic effect with purple, green and blue reflections as seen in the blue moissanite, or a strong golden colour as seen on the yellow-brown stone (Figure 4), on pavilion facets in reflected light. Furthermore, the pink stones, in particular, showed a mottled appearance on the surface, with a dotted, sputter-coated effect (Figure 5) typically seen in surface-coated topaz and surface-coated quartz, together with a subtle ‘sandpaper’ texture observed under low levels of light reflection (Figure 6).
Figure 2. Two circle reflections, referred to as ‘doubling’, seen to the right of the culet when viewing moissanite through the table facet.
Figure 3. Surface scratches observed on the pavilion facet of a coated moissanite under reflected light.
Figure 4. Metallic effect of various colours on the pavilion of surface-coated moissanite.
Figure 5. Dotted, sputter-coated effect on the pavilion facet of a pink surface-coated moissanite seen under reflected light.
Figure 6. ‘Sandpaper’ texture on the pavilion facet of a pink surface-coated moissanite under low reflected light.
Accompanying Certification of Moissanite
These surface features are consistent with treatments with metal coatings commonly used to change the appearance of stones, such as topaz (Schmetzer, 2008). Whilst the treatment itself is not new, what is disturbing about these moissanites is that they are being sold with GRA certificates that have been modelled on the GIA Diamond Grading certificates. There are similarities between the certificates in colour, font, branding and layout, complete with proportions and cut grading (which are not applied to moissanite; Figure 7).
The certificates are generic and virtually identical except for the apparently unique certificate number. There are some glaring spelling mistakes, for example, “dis clalmers” rather than disclaimers, and “Gemplogy” rather than Gemology. The measurements of the stones are approximate and quoted as calibrated ideal measurements, but the actual stones show slight variation. The carat weights are also approximate with a disclaimer which reads “* Diamond weight in same size Real weight:”, to suggest the carat weight quoted is the Diamond equivalent of the same measurement. The moissanites, because of their lower specific gravity compared with diamond, weigh approximately 10% less than the carat weight printed on the report, depending on cut proportions. There are additional spelling, grammatical, factual and historical errors in the “About GRA” section.
The laboratory, “GRA”, is an abbreviation of “Global Gemological Research Association”. Each stone comes with a numbered certificate with this same number also laser inscribed on the girdle (Figure 8). The laser inscription on the stone and certificates match, as do the details on the bags the stones come in. Each of the twenty stones purchased had unique certificate numbers. The QR code on the certificate takes you to the website for GRA, and, when accessed by the author, there is an alert by the website browser claiming this is not secure. The certificate number appears to bring up the same details as printed on the certificate, that is, the stone colour and size match. It is not known how unique these numbers are, nor how many identical colour and size stones are produced with the same number. Interestingly, while the report clearly identifies the stone as moissanite, the surface coating treatment is not mentioned, and GRA does not use the word “synthetic” on the certificate.
In the interest of ongoing member education, the GAA (Victoria Division) has added these stones to the teaching collection. They sit alongside an increasing number of surface-coated natural and synthetic stones that include topaz, quartz, cubic zirconia and diamond.
Figure 7. GRA certificate that accompanies the surface-coated moissanites purchased.
Figure 8. Laser inscription on the girdle of the pink moissanite.
Reference
Schmetzer, K., 2008. Surface treatment of gemstones, especially topaz – an update of recent patent literature. J. of Gemmology, 31(1,2), pp.7-13. DOI:10.15506/JoG.2008.31.1.7
