Synthetic Moissanite

The synthetic form of the silicon carbide moissanite, SiC, has been manufactured for ornamental and gem use. Most of the properties of diamond are quite well imitated and the usual anxiety associated with new diamond imitations was reported to be pervading the trade. Such reports are usually exaggerated the properties of moissanite may be detected easily by a gemmologist with simple equipment.

Synthetic moissanite belongs to the hexagonal crystal system and shows birefringence absent from diamond though doubling of back facet edges cannot usually be seen through the table facet, which would have been eas- ier to detect, but in a direction at right angles to this. Near-parallel needles and stringers may be seen at right angles to the table. Some specimens show rounded facet edges (those of diamond are exceptionally sharp) which are not in themselves a vital clue. There are also some uni-directional polishing lines on adjacent facets, which do not occur on diamond.

Gemmological properties are:

• Hardness: 9.25
• Refractive Index: 2.648–2.691 with a birefringence of 0.043
• Uniaxial positive and dispersion: 0.104, which is more than twice as great as diamond
• Specific Gravity: 3.22 (diamond is 3.52)

These properties can all be tested with a little effort but with any diamond imitation it is usually worth devising a catch all detector. Reflectivity meters have usually been used to separate diamond-like transparent stones from their more serious imitators YAG and CZ though they can only reach a few stones in a piece of jewellery which contains many small ones in hard-to reach places. These lurking ‘diamonds’ can more often be reached with the thermal conductivity tester which will very effectively separate diamond from most of its simulants.

It has been suggested that manufacturers of synthetic moissanite might be able to alter the RI of their product so that the reflectivity meters might give a ‘diamond’ reading. [Reflectivity meters do not meas- ure RI as such but RI does influence reflectivity.] In such a case it might have been possible to repolish the specimen so that the original RI could be assessed. Rumours of this kind often circulate through the trade and those involved should ask themselves ‘what might the manufacturer gain from all this trouble?’

A synthetic near-colourless moissanite has been heat-treated, the treat- ment causing a brownish colour to develop across all the facets. Cleaning and hand-polishing the samples, using cerium oxide on leather, restored the reflectivity to 98% of the non-treated material. If heating forms part of any testing experiment on a suspected moissanite, the gemmologist should remember that surface oxidation could occur and keep the level of heating to a minimum. The colour of the surface might undergo alteration.

The thermal conductivity tester will give a ‘diamond’ reading for syn- thetic moissanite in any case so that its use may be confined to separating diamond and synthetic moissanite from other diamond simulants: a further test to separate the two could well be magnification. Diamond will sink and moissanite float in di-iodomethane (SG 3.34).

Some coloured moissanites have been brown, green, yellow and blue but the colours are not very strong. Green specimens that I have seen are not like green diamond. A brown moissanite grown in Russia is reported to have been grown by CVD: the specimen described was opaque.

The first firm to produce synthetic moissanite and sell it (exclusively, at first) was C3 Inc. in North Carolina, USA. The same firm, now called Charles & Colvard, produced a tester, which they sold under the name Tester Model 590. Other instruments have followed. These separate moissanite from diamond by scanning the blue and near-visible UV areas of their absorption spectrum.

Diamond and Moissanite Testing

Testing if a specimen is a diamond or moissanite can be done using different instruments and techniques.

Halogen Light Source

In moissanite there is an intense region of absorption extending down from about 425 nm to the UV region. Colourless diamond, on the other hand, transmits well down into the UV. A halogen light source directs a beam on to the table facet which reflects it. If it transmits wavelengths from the blue to the UV region the tester gives a visual indication plus a bleep, indicating that the specimen is diamond. If no response is given the specimen will be moissanite, having absorbed this range of wavelengths.

Presidium Moissanite Tester

Another instrument, the Presidium moissanite tester, detects the very small current passed by semiconducting materials. The operator receives a signal indicating diamond/not diamond. As most diamonds, apart from type IIb blue diamonds, are not semi-conductors, any current detected may be caused by impurities in the material. In a paper in Australian Gemmologist 20, 483–85, 2000, the authors found that the testing of synthetic moissanite was made easier by the Presidium tester. As synthetic moissanite is a semiconductor, the instrument is able to sense a forward leakage of current. All synthetic moissanite specimens were detected as such by the apparatus, indicating them by the illumination of a bright red window display and a sound alert. False positive synthetic moissanite readings occurred during the evaluation, particularly when the tip of the probe was in contact with metal (such as the setting) and also when a germanium transistor was touched. A synthetic moissanite response was also given by a black electrically conductive industrial-quality diamond.

Other Techniques

Another trick is the application of a film of blue or violet dye on the back facets of the stone: the blue or violet and the yellow colour of the stone combine to give a whiter effect. The film is thin enough to escape notice except where it has caught up on the raw edge (the unpolished girdle or setting edge of the stone). As the treatment is usually carried out by using a water-soluble dye, a thorough washing in hot water will dissolve the colour. Should a coloured lacquer be used, solvents such as acetone or amyl acetate, or even acid, may be needed to remove the colour. Similarly pink diamonds have been imitated by painting the back facets with a pink dye or enamel.

Synthesis and Simulation

Synthetic diamond is described in the next chapter.

 

Imitations of Diamond – Natural Stones

The high lustre and dispersion of zircon would make it an excellent imita- tion of diamond though it is not very hard and its high birefringence would easily be detected. More difficulty can arise when a coloured natural stone is mistaken for diamond. A search of the descriptions in the relevant chap- ters may lead readers to find out for themselves which ones are most likely to cause confusion.

 

Imitations of Diamond – Man-Made Stones

Descriptions of synthetic rutile, strontium titanate, the synthetic garnets YAG, GGG and their analogues, cubic zirconia and synthetic moissanite are described in the next chapter. Composites are described in the next chapter.

Details of testing instruments and how they are used can be found in the 3rd edition of Peter Read’s Gemmology, 2005 (ISBN 0750664495

Introduction

On average, approximately 1 tonne of rock from a diamond mine must be processed to recover each carat of diamond. In other words, the diamond concentration is about one part in 5 million. Even then, less than 25% of the material is of gem quality. However, once a gem diamond has been found, what a prize it represents! As the hardest known material, it can take a polish unsurpassed by any other gemstone; its transparency in the visible region, its high RI and high dispersion result in a brilliance and a fire that are truly distinctive.

Uniquely, diamond is the only gem material comprised of a single chemical element; pure diamond is made exclusively of carbon. Following the research in the late eighteenth century by Antoine Lavoisier, Smithson Tennant and Humphrey Davy, who, amongst them, first recognized that diamond was simply one of the possible forms of carbon, there followed many attempts to synthesize diamonds in the laboratory from one of the other, less valuable forms of carbon. Some of the researchers who attempted to manufacture diamond concluded that, because natural dia- monds were probably produced under geological conditions of high pres- sure and high temperature (HPHT), such conditions would be necessary to manufacture diamonds in the laboratory. However, it was not until the middle of the twentieth century that the first undisputed synthetic diamonds were produced.

The original process for synthetic diamond produced grit-sized particles suitable for industrial applications, but developments of that process now allow HPHT synthetic diamonds weighing a few carats to be grown on a commercial basis.

Of far greater concern to the gem trade is the fact, made public in 1999, that the colour of some natural diamonds can be enhanced by HPHT pro- cessing of natural diamonds. In particular, certain brown diamonds can be converted to near-colourless, or occasionally pink or blue colours. If suitable starting material is available, it is far more lucrative to use HPHT equipment to enhance the colour of natural diamonds (which takes only a few minutes) than to tie it up for days growing large synthetic diamonds. Within the last few years a new method has been developed for pro- ducing gem-quality diamond, using chemical vapour deposition (CVD). This does not require high-pressure presses or extremely high tempera- tures. Instead, a carbon-containing gas is decomposed at a pressure some- what below atmospheric pressure in an energetic plasma, and the carbon

is deposited as diamond.

This progress in diamond research therefore presents the diamond gem trade with a number of unwelcome developments:

•    It is possible to produce gem-quality diamonds by HPHT synthesis, on a commercially viable basis. Depending on the growth process, and subsequent treatments, the diamonds can be grown in a wide range of colours.

•    Equipment designed for HPHT synthesis can also be used to enhance the colour (and therefore the selling price) of natural brown diamonds; this appears to be more commercially attractive than using the equip- ment to grow synthetic diamonds.

•    Gem-quality diamonds can be produced by CVD.

At present the quantities of HPHT synthetic diamonds, and of HPHT colour-enhanced diamonds, form a very small fraction of the total market. The production of CVD diamond gems is in its infancy, and the economics of the process are not yet well established. Despite the small numbers of these relatively new forms of diamond, gem testing laboratories will nevertheless be expected to detect such specimens when they turn up. In some cases, a simple visual inspection will suffice, in other cases sophis- ticated spectroscopic techniques will be required.

Once regarded simply as curiosities, during the last decade coloured diamonds have begun to come into fashion, and to create their own market. HPHT synthetic diamonds can be produced in a range of ‘fancy’ colours, the HPHT processing of natural brown diamonds brings about a dramatic change in the colour of the diamond and CVD diamond fre- quently has a brown colour which can be enhanced by HPHT processing. It is therefore vitally important that we have a reasonable understanding of why some diamonds are coloured. In fact, the vast majority of natu- ral diamonds are coloured, which is why a top-quality near-colourless stone can command such a high selling price. In the following section of

 

this chapter, we shall explore the current understanding of the origins of colour in diamond. The HPHT and CVD processes will then be consi- dered, and we will conclude by examining methods by which naturally coloured natural diamonds can be differentiated from other diamonds which are increasingly finding their way into the market.