The Scottish Gemmological Association

 

 

 

 

Synthetic Moissanite, Diamond and some distinctions

by Alan Hodgkinson

 

Scottish Gem Lab News June 1998 updated 16th November 1998

Uploaded to the SGA Website on January 17th 2010

(Click on thumbnails to view pictures.  Close the window to return to the text.)

 

Table 1.  Two serious trade contenders for a diamond-like appearance
Key distinguishing factors RI DR Dis H SG Luster*
Moissanite 2.7 0.04 0.11 9+ 3.2 5
Diamond 2.4 nil 0.04 10 3.5 4
CZ 2.2 nil 0.06 8+ 5.7 2.7

Other diamond simulants are not included, as they are too rarely met with, far too soft, or do not pose a sufficiently diamond-like identity.  Synthetic diamond has the same optical properties as diamond and cannot, therefore, be distinguished by the methods discussed in this article.

* The numerical values for "luster" are taken from the table of "Hanneman Relative Luster Values", Summer 1977, Gems and Gemology, pages 302 to 305, and are easily demonstrated on the Hanneman  "Diamond Eye" (see Fig. 23)
 

1: Diamond and some higher refractive simulants.
Which is which? Can you identify the various colourless gem identities seen? Answer after Figure 52.
 
2: Main Colours
Green hexagonal silicon carbide crystal (synthetic moissanite) alongside faceted green, yellow and colourless specimens.

3: Relative Hardness

The synthetic ruby shows scratch marks on the table facet from moissanite because the hardness of this silicon carbide material (9+) is greater than corundum's (9) on the Mohs Scale of hardness. Hopefully this will see an end to such a test, formerly deployed by some for identifying diamond (10).

4: Thermal Probe

The thermal probe had become a great friend of the jewellery trade, because of its ability to separate diamond from cubic zirconia, even when set in jewellery. A scale model, as here, is even more useful as the pointer shows a dramatic surge with diamond's high conductivity, compared with almost no response from CZ, which acts almost as an insulator. A thermal probe responds to synthetic moissanite in the same way as diamond, and so is unable to distinguish between the two. A solution to the problem will be found among some of the gemmological tests described below.

5: Double Refraction

The culet invariably appears singly refractive through the table facet, while the refracted image of the culet region shows double refraction when seen through the main crown facets, as here. The occurrence repeats under 8 of the main crown facets as the stone is manoeuvred.
With such evidence, it is more than obvious to any thinking gemmologist that here is a birefringent mineral, which distinguishes itself from diamond by a x10 loupe.  "What's the problem?"  I hear you ask.  Yes, but what if your loupe has been borrowed by another member of staff at the critical moment of examination?
The Hodgkinson Method separates the two, even in the dark, without looking at the stone, without touching; even if mounted, without instrumental aid other than a flashlight torch.  Even a candle or the moon will suffice, if available!!  What's more, it's good for the soul of a keen and enthusiastic gemmologist, but make sure you get your loupe back.  The next problem may not be so easy.

6: Table Reflection Doubled

Culet single, table reflection doubled. Observation by Howard Rubin (Inventor of Gem Dialogue). Method - focus on the culet, then lower the focus of x10 lens or microscope, below the culet, when a small table facet reflection will be seen surrounding the culet area. The reflected table has a doubled outline in the manner of a picture frame; thus confirming birefringence.
7: Star Facet Reflection Doubled

Focussing further down just below the table reflection, the star facets come into view, and these also show "doubled" outlines. Features in Figures 6 and 7 are shown in Gems and Gemology, Winter 1997, page 268
 

8: Obvious Dispersion

The high dispersion (0.11) is apparent in this larger moissanite (1.26ct) and readily distinguishes it from diamond (dispersion 0.04) in this size of gem.
 

9: Dispersion Missing!

The dispersion (fire) distinction of moissanite from diamond is not outwardly evident to the eye in small stones, especially when set in an eternity ring for instance, and even less so if the stones are not clean. The eternity ring is set with small, princess cut synthetic moissanites. Set in jewellery, these newcomers will present a problem for the jewellery trade once they start returning to jewellers' shops for various services, especially after day-to-day wear, when the pavilion surface becomes "organic"!   Synthetic moissanite will help force the pace of more and better staff training in basic gemmological skills. The eternity ring (courtesy Jeff Hunter and Howard Rubin) was easily distinguished from diamond by raising the ring to the eye and observing its "Visual Optics" performance - see Figures 29 and 30.

10: "Silk"

Often seen in moissanite are fine hair-like inclusions reminiscent of "silk" in Sri Lankan sapphire. Though not all parallel, there is a general direction to their arrangement which follows the "c"-axis.
 

11: Criss-cross and "Morse Code Silk"

In some moissanites the silky inclusions bifurcate, some stop and start along their length, some angle at one point and then resume their original direction (geniculate fashion). All the silky variations mentioned would seem to be fine tubular versions of those in Figure 12.
Some of the silky inclusions featured here have a similar aspect to the diamond photomicrograph featured by John Koivula on page 72 of "A photolexicon of inclusion related terms for to-day's gemologist".  Canadian Gemologist, Autumn 1998, pages 67 to 74.

12: Tubules

The rod-like inclusions are simply coarser versions of the silk in Figure 11, and they are, in fact, tubules, as can be seen where they come to a facet surface. Some of the tubules also bifurcate.  In the main, again, many of these tubules are parallel to the "c"-axis, or nearly so.  It should be borne in mind that the repertoire of inclusions in diamond is vast.  They can mimic the appearance of inclusions in other gem materials, so one should always be cautious when evaluating inclusions with a x10 loupe.  See Fig. 42.

13: "Bicycle Wheel"

There are so many tubules in this stone that they appear (by reflection across the pavilion) similar to the spokes of a wheel. In fact the tubes correspond closely to the "c"-axis, although this is not apparent from the photo.
 

14: Solid Inclusion

A smoke-like structure flows from what appears to be a crystalline inclusion.
 

 

15: "Creases"

One stone seen has interesting surface marks resembling creases. As these were continuous across several facets, it was presumed they were similar to grain lines in a diamond, which represent hardness/softness boundaries. In one zone the lines were continuous, while in an adjacent zone they were discontinuous dots and dashes with "morse code" appearance.

16: Cubic Inclusions?

Rounded crystal inclusions in a brownish/yellow moissanite were so compact in form as to suggest crystallization in the cubic system.
 

 

17: Electron Microscope View (Courtesy Dr. Placido, Paisley University)

Analysis of the inclusions in Figure 16 showed silicon, carbon and vanadium to be present. This, and the compact form of the inclusions, suggest the beta form of silicon carbide which is isotropic, but this is only speculation.

The vanadium may derive from the petroleum used in the synthesis being rich in the fossil remains of the shrimp-like creatures - holothurian- which store vanadium in their blood. This suggestion came from Dr. J. Nelson, who had experience in growing silicon carbide crystals circa 1948.

18: Diamond Ultra Violet Fluorescence

This shows typical sky-blue fluorescence of a cape series diamond under long wave ultra violet light. A brown series diamond alongside fluoresces greenish yellow.

 

Moissanite Ultra Violet Fluorescence - Long Wave

The thirty-five colourless moissanites tested (courtesy Moissanite UK Limited) appear to fall into one of two colour grade categories.
a) greenish grey specimens which fluoresce a cinnamon orange shade.
b) yellowish cape series which show little observable fluorescence.
N.B. Occasionally diamonds are seen fluorescing in a similar cinnamon colour under UVLW.
UVSW. The moissanites tested by the writer showed little observable fluorescent response to short wave UV light.
Research Director Dr. Mark Kellam at C3 Inc. took a piece of colourless synthetic moissanite crystal which showed little UV fluorescence.  After cooling in liquid nitrogen, the material fluoresced a very visible orange under the ultra violet lamp.

UV Fluorescence - other possible identities

Though not often seen, GGG fluoresces a dull orange under UVLW.  However, it responds unusually with an even brighter yellowy orange under UVSW, and there is a lingering brown colour to the stone.  This is not phosphorescence.  The brown tinge finally disappears, but is aided to do so by slight heat.

Electroconductivity

The green and dark bluish black specimens are all electroconductive. Colourless to near colourless specimens vary in their electroconductive response. Some of the yellow tinged stones showed electroconductivity. So far I have not seen one of the greenish grey moissanites conduct, and would welcome any contradiction of this observation.

19: Colour Grades

The row of stones shows a central "F" colour diamond of 1.00ct with three smaller moissanites on the left having a "cape" series aspect. On the right of the diamond are seven smaller moissanites with a slight greenish grey cast.

20: Spectrum

The top spectrum of diamond shows a distinctive 415nm line, as observed in many natural diamonds. Lesser strength lines may or may not readily show in the region. The lower spectrum of moissanite shows the cut off from approximately 425nm. The spectra on the left are by diffraction. The spectra on the right are via a prism spectroscope.

21: Ultra Violet Probe

The C3 Model 590, developed by the American company, C3 Inc., which is growing the colourless hexagonal synthetic silicon carbide. The probe works by distinguishing that the diamond transmits light in the ultra violet region of the spectrum, while the moissanite absorbs light from the violet into the ultra violet. This can be anticipated from their absorption spectra in Fig. 20.
The thermal probe (Fig. 4) was developed mainly to separate diamond from CZ in the early 1980s. Unfortunately, diamond and synthetic moissanite are both good thermal conductors and cannot readily be separated by thermal probe.

The "C3" strategy is that, when  a thermal probe indicates diamond, the stone should then be tested with the ultra violet probe to distinguish between diamond and moissanite.  Objections have been voiced that the UV probe does not always work.  Dr. Kurt Nassau responds that the probe does always work, though the result is aided by moving the probe tip about over the facet surface.

As an afterthought for the keener gemmologists, cassiterite (colourless and browns) will also register as a diamond on a thermal probe, and identify as a moissanite on the UV probe. The cassiterite RI is 2.0 - 2.1, Dispersion 0.07.,  DR of 0.1, which is twice that of moissanite's 0.04.

Keep an eye open, as they both appear in yellowy/orangy/brown shades of colour as in Fig. 2. Visual Optics readily picks out the main primaries of the lower refractive cassiterite.  In fact they are rather reminiscent of zircon, Fig. 32.

22: Japanese "Culti", Coloured Stone Checker

By chance I have one of these instruments, and switched it to its UVLW transmission test facility. The stone immediately registered positive for diamond, but gave no reaction to the synthetic moissanite, as could be anticipated from the absorption spectra at Fig. 20.

For those with such an instrument, there is a provision for distinguishing emerald, ruby, sapphire and alexandrite from some of their synthetic counterparts. Needless to say, there is no setting for synthetic moissanite. After some experimentation, the distinction between diamond and synthetic moissanite could best be achieved by turning the instrument to the "emerald" setting, though the sapphire setting also provided an instant separation between the two. The Culti will not cope with tiny gems, nor smaller set stones, as can the C3 590 probe.

It is five years or more since I last used the Culti coloured stone checker, but it is interesting to find that the instrument stood by its principle design function, by distinguishing moissanite from diamond.

23: The Hanneman Diamond Eye

The Hanneman Diamond Eye, is a  reflectivity meter, which creates a scale sequence of reflection. This  is comparable to the refractive index order.  The inventor described it as a "Lustermeter" in Gems and Gemology, Summer 1977, p302 to 305. Even then, the 1977 article mentioned the possibility of identifying, or eliminating, silicon carbide.
No contact liquid is required, and the instrument copes with gems refracting as high as diamond, rutile or synthetic moissanite. The instrument was devised in the 1970s to cope with the contemporary threat of YAG  "Diamonair".

For those with earlier Diamond Eye models, which are not marked for moissanite, the friction fit plastic hood lifts off carefully to allow code "5" to be added for synthetic moissanite,  beyond diamond (code number "4") as shown in the photo. Similarly in the 1980s an "x" could be added to mark the CZ position on the scale of  earlier models.

A similar, but more expanded version, called "The Jeweler's Eye" includes two separate scales, one for a fuller range of higher refractive gems, and a lower scale for those gems under RI 1.8. The lower scale cannot, of course, compete with the information obtained from a refractometer. The Diamond Eye can be hindered by jewellery settings and is, therefore, sometimes restricted to larger mounted stones. The C3 590 Probe is not restricted in this way, but both instruments also have a wider gemmological role to play.

STOP PRESS Dr Mark Kellam has been researching ways of changing the surface structureof synthetic moissanite.  As a result he has been able to change the reflective power of the surface. The change can be doctored to record any surface reading as low as quartz and less.  C3 inc have kindly given me a one carat altered synthetic moissanite which reads daimond on a reflectance meter.  Such an alteration could be disconcerting if it were not the fact that using the Hodgkinson method, the difference between diamond and moissanite is instantly seen as shown in Figures 29 and 30.
Reflectance meters and refractometers work only on the surface presented to the instrument. Visual optics literally sees through the stone to reveal the truth of the body of the stone

24: Comparative Density and Optical Relief

The set-up shows a glass cell containing methylene iodide, (SG 3.3). The moissanite (SG 3.2) can be seen floating. At the bottom from left to right can be seen a submerged diamond (SG 3.5), YAG (SG 4.5) and CZ (SG 5.7). This is described as "Density Separation".

Simultaneously it is seen that the optical relief, when submerged in this liquid, is directly related to the difference between the RI of the stone and that of the methylene iodide (RI 1.74). The most obviously visible stone in the immersion fluid is the floating moissanite (RI 2.69, refractive index difference 0.95), followed by the diamond, bottom left, (RI 2.42, RI difference 0.68), then CZ, bottom right, (RI 2.17, RI difference 0.43). Finally the YAG which, with an RI of 1.83 and RI difference of 0.09, is barely visible.

A colourless synthetic spinel would completely disappear in the liquid due to its RI of 1.73, very close to that of the methylene iodide (RI 1.74), an RI difference of 0.01.

25: Conoscopic Observation

The conoscope reveals a uniaxial figure on the polariscope. This rules out diamond, but does not, of course, prove that the stone is moissanite (zircon, rutile, cassiterite and lithium niobate, etc. would do the same, as they are also uniaxial positive, as it happens).

The figure tends to be "scrambled" by the excessive brilliance of the stone due to its high refractive index, but it can still be made out quite simply. The figure is seen more easily the smaller the concoscopic sphere.  Such a small sphere requires magnification to see the figure.  Dr. J. Nelson showed me a small telescopic device which sits atop the analyser for that purpose.  A microscope or x10 loupe will also help resolve the figure.

Such a diagnostic feature can invariably be seen even though the stone is set in jewellery, because moissanites are usually cut with the table perpendicular to the "c" axis of the crystal.

Polariscope

Rotation of the moissanite on a polariscope will, of course, provide the fourfold contrasting illumination/extinction response.  This alone is sufficient to indicate double refraction and thereby rule out diamond.  The effect is generally seen through the girdle, so this will pose a problem with certain mounted stones.

26: Optical Sign

By means of a mineral accessory plate, such as mica, gypsum or quartz wedge inserted above the stone, it can be shown that the stone has a positive sign. In this case, the two dots, which represent the ordinary and extraordinary rays, line up across the direction of the introduced mica plate to indicate uniaxial positive. An inexpensive substitute accessory plate is available for this purpose and sold as the "Hanneman/Daly quartz wedge simulator".

27: Visual Optics

Visual Optics, or The Hodgkinson Method, easily distinguishes between diamond and moissanite, or any other diamond simulant - see book or video, "Visual optics - The Hodgkinson Method". The stones can be loose, set, even solid wall set provided there is an open back. Furthermore, the stones can be large or small, but reasonably clean they must be.
Some examples of the distinguishing patterns are shown as follows:-
28: CZ
As the RI of cubic zirconia is approximately 2.2, there is a little leakage of light in a brilliant cut, which reveals about three main "Primaries", whether the stone is large or small. Any small "Primaries" or "Secondaries" seen, lack the compact sharpness of diamond's performance.
29: Diamond
With RI 2.4, a brilliant or princess cut diamond is able to exclude the light source by total internal reflection. As a result, no main facet large  "Primaries" are observed, but identification is confirmed by a swarm of small sharp spectral " Primaries" and "Secondaries", etc. A very shallow brilliant or a step cut diamond will reveal large main facet "Primaries", but identification is again made possible by the distinctive small sharp spectral "Secondaries etc".
30: Synthetic Moissanite
With a RI of  2.7 (double refraction 0.043), total internal reflection takes place as with diamond, and no large main "Primaries" are visible. The small "Primaries",  "Secondaries" and subsequent reflections are visible, whatever the size of the stone, and whether loose or set, appear like ragged spectrum flakes against the blacked out view. The effect is so kaleidoscopic as to make the birefringence quite difficult to decipher, but the distinction from diamond is immediate and obvious.
As an experiment, I quickly taught ten year old Joshua Pilling from Lincolnshire how to distinguish diamond from synthetic moissanite, in the dark, without seeing the stones, without touching them, and without any other aid than a candle or flashlight torch. (The teaching process took two to three minutes). Joshua was then able to teach his father, David Pilling, how easy it is to distinguish diamond from synthetic moissanite - in the dark, without seeing and without touching the stones!!!

It is actually easier if the stones are set, as the finger tips can hold the side of the shank or whatever. Incidentally the pattern for strontium titanate is similar to moissanite, but even more diffused. The softness of strontium titanate (5 on Mohs Scale), and its lack of double refraction, easily separate it from the hard moissanite.

31: B/D Ratio

The high RI 2.7, and the optic axis at right angles to the table facet make it awkward to locate the double refraction by Visual Optics. The photo which captured a doubled image was obtained through the girdle area of the stone while looking towards the light source. It will be seen that the B/D ratio is just under half (i.e. DR 0.04 divided by dispersion 0.10 = 0.40). This means that the doubled spectral images are never separate.

32: Zircon (high type)

The RI 1.93 - 1.99 generally causes three doubled primaries to come into view in a round stone. Their highly spectral appearance is quite dramatic with the DR 0.059, the dispersion 0.037. The resultant B/D ratio, of approximately 1.5 at its maximum, literally separates the doubled spectra, which is in marked contrast to the part superimposed  moissanite images.
The above characteristic views can be achieved with a fibre optic light about four feet distant or more, but more entertainingly in the dark with a candle, flash light torch, or even a bright moon as advocated by Dr. J. Nelson.

The B/D ratio is the brainchild of Dr W (Bill) Hanneman, as described in the book and video mentioned. More recently Dr. Don Hoover has written a very in-depth article on the "Hodgkinson Method" in which he explores the whole B/D area very fully - see Australian Gemmologist 1998 Vol. 20, pages 20-33.

"Primaries" in synthetic moissanite

Large main  pavilion facet Primary images are lacking in faceted moissanite and well proportioned round diamond, though they are seen in CZ Fig 28 and zircon Fig 32. This is simply because the high refractive power of the first two mentioned will not permit the light to pass through the pavilion  as  refracted "Primary" images.  The high RI causes the light from the small girdle facets to bounce about within the stone in the form of small "Primaries and  "Secondaries".

Wishing  to produce such a large primary image in moissanite so that optical measurements could be undertaken, I asked Doug Morgan to cut me a shallow prism to replicate the function of the pavilion in a faceted gem. Successive experimental cuts were attempted to enable the light to pass through, but it was not until the prism (pavilion) angle was 30o, that the light could pass and a complete primary image could be examined.

From the previous paragraph it will be seen that the cutters of moissanite could achieve a useful economy. At the moment, the larger sizes of moissanite (above 8mm) are rare. Cutting the stones with a shallower pavilion would mean a wider stone from a shallower crystal. The resultant stone would still have the brilliance of a diamond, even at a pavilion angle of 35o. In a diamond, this would present a very "fisheye" stone.
Earl Hines controls the cutting and polishing of moissanite at The North Carolina plant of C3.  He is at pains to design such cuts of the material as will show off its optical power to the full.

33: Primaries in a brilliant cut diamond

The large main "Primaries" are not seen in a brilliant cut diamond because the high RI of diamond and synthetic moissanite block them off by total internal reflection.  The facets captured in Fig. 33 are in fact such "Primaries".  How  were they captured?   In 1984 Dr J Nelson of London built a Pavilion Facet Fingerprinter, a gem projection instrument which achieves three benefits:-
a)    every pavilion facet can be plotted;
b)    enables teaching of the Hodgkinson method;
c)    allows comparison of one gem variety's optical behaviour with that of another gem variety.

To overcome the total internal reflection of higher refractive stones, Nelson immersed the stones in benzyl benzoate at RI 1.57.  This enabled the primaries to be observed, even though they are normally beyond reach  by Visual Optics.

Here a diamond displays its primaries  in baby oil, RI 1.47.  while Water, RI 1.3, works equally well.  The baby oil recommendation comes from geologist, Pat Daly, and makes a much safer fluid for general horizontal microscope study of gemstones.  Note how the diamond dispersion of 0.044 is virtually unnoticed.

34: Primaries in a brilliant cut synthetic moissanite

Under the same conditions as diamond in the previous Fig. 33, the higher dispersion of synthetic moissanite, approximately 0.10, shows an extraordinary display of technicoloured dispersion.  This is yet another way to separate diamond from synthetic moissanite.  See Fig. 43 for diamond parcel screening.
 

35: Heat resistance

Because synthetic moissanite has remarkable heat resistance, due in part to its excellent thermal conductivity, it can withstand temperatures higher than diamond. Consultant to C3 Inc., Howard Rubin, explained to me that the stones can in fact be set in a wax model such as an eternity ring, then cast in 18ct gold . The gold casting emerges with the stones already set in situ, which offers a great saving in time and trouble, especially for channel set or rubover settings.
An 18ct gold ring has been melted on a black charcoal block.  The synthetic moissanite placed in the molten gold immediately turns yellow.  The gold melting  temperature is approx. 900oC
 

36: Retipping (Repronging) a synthetic moissanite ring.

The 18ct white gold ring mount was rescued from a gold scrap box, and set with a 5mm synthetic moissanite. One claw was deliberately broken off, and a large 18ct. gold retip applied purposely. No heat guard agency was employed, and no damage to the stone resulted.
Diamond would be damaged by such a procedure, unless care was exercised along with a recognised heat protection agent, e.g. boric acid  and alcohol (USA) or borax and water  (UK).

37:  A heat damaged diamond

This marquise cut diamond has been completely ruined by careless use of excess heat in a claw repair operation. A skilled jeweller would have controlled the gas torch so that the temperature was restricted.  If not taken from the setting, a suitable heat guard should be applied to the diamond to protect the stone from the heat. The trade variously refers to such a damaged diamond as "smoked", "burnt", "milked" or "fired".

38:  A heat damaged synthetic moissanite

Yes, it is possible to burn the surface of a moissanite in a jewellery workshop.  However, this was only achieved by melting 18ct gold, fluxing with borax and retaining the stone in the molten gold for three or four minutes.  Such exceptional treatment would not be met with in a casting or retipping operation.  A diamond would simply not survive.

39: CZ damaged by workshop retip operation

CZ would normally shatter under the heat of such an operation, as here, which makes it troublesome for various jewellery repairs.  Sustained workshop torch heat to CZ can actually turn the stones to various orangy shades, which remain permanent.

 

Facets and girdle

Any attempt to pre-empt the distinction of diamond from synthetic moissanite on the grounds of the facets, or the girdle, is liable to mislead those who are faced with such identification in the future. Facets are as good as the facetor.  While the facet edges of a poorly cut moissanite will show a "roll over" effect normally unseen in diamond, the facets of a non-diamond such as CZ, or even more so moissanite, can be "diamond sharp" if in skilled hands, and time allows.
A diamond girdle can be rough ground, smooth ground, faceted or polished (the latter an invention of the British diamond cutters - Monnickendam). There is of course nothing to stop a gem cutter faceting or polishing a synthetic moissanite girdle.  C3 Inc. are polishing moissanite girdles to gain even more life from the stone, as is the aim with diamond.

40: "Bearding"

Although we are taught that "bearding" on the girdle of a diamond is a derogatory feature, the fact that it is unique to diamond makes it a useful identifying observation. This is what I term a "commercial gemmological clue", that is economic factors of time and cost make bearding more and more likely,  and it will instantly separate diamond from moissanite or CZ.  A great many round diamonds afford this clue to-day.
 

41: "Cups and Saucers" on the facet edges of the girdle

This microscopic feature (x56)  is often seen on synthetic moissanite and CZ, and stems from the brittleness of the material in response to the girdling (bruting) operation.  Improving awareness and technique are overcoming the problem.
 

42: Deceptive Inclusions in diamond

It would be easy to jump to conclusions and class these parallel lines as being similar to the tubules in synthetic moissanite Fig. 12.
 
 

Diamond Parcel Testing

Faced with a number of diamonds in a stone parcel, the question arises, "Are they all diamond, or are there any moissanites, or CZ?"

Imagine a parcel of 300 diamonds salted with, say, 3 moissanites.
Imagine a tennis bracelet of 40 diamonds with a moissanite rogue.
Imagine a parcel of 300 moissanites salted with, say, 3 CZs.
Answers to these problems can be gained in under one minute, by one of the following routes.

43: Tables Down

Using the Nelson Fingerprinter, round stones lying table down cast their primary image circles. (Other shape cuts will display their relative shape.)  The outer technicoloured circle indicates moissanite. (See Fig. 34)  Two inner circles of whitish light images are seen.  The larger of the white image circles is diamond (Fig. 33), the smaller white image circle is CZ.  Closely similar diameter circles occur for each identity regardless of the stone diameter, provided the pavilion angles are similar.  It is all relative to the optical indices quoted in Table 1, and the Hodgkinson Method..  Furthermore, other gem identifications or eliminations are obviously possible.

44: Pavilions Down
Here the three round stones are lying on their pavilions.  They cast only part of their outline, but it is ample.  The pyrotechnics of moissanite again stand out.  Hundreds of diamonds can be screened for moissanite in a minute or so.

45:  Nelson Fingerprinter

This intriguing piece of equipment is ideal for laboratory or appraisal use to provide a unique observation of important gems.  A photographic record can be made, not only of the pavilion facet plan, but also the tiniest deviations and irregularities in facet placement which show up.  Photographs are easily taken, and would make a useful archival record of a valuable gem.  Such a record would also provide a "fingerprint" in forensic work, as it would distinguish one diamond, ruby, emerald, etc. from any other.  The Nelson Fingerprinter is obtainable from Nelson Gemmological Instruments, 1 Lyndhurst Road, London NW5 5PX.

 

Optional Method


Pour a diamond parcel into a clear plastic lid or box and cover with water.  Support the box above a white A4 paper, which acts as a screen underneath.  The box is best supported on a small glass shelf about 1" above the paper, so that light can pass down through the stones to the paper.  (The glass from a small, cheap photo frame about 10" x 8" is ideal.)  Position a fibre optic, or hold a pocket flashlight torch, about 6" above the box.  Simply move the clear box about laterally on the glass shelf and below the light source.  Improved performance comes if you cover the light with a disc in which you cut a fine slit.  Best results are obtained in the dark.  Trawl a pair of diamond tweezers through the stones.  Any moissanites will quickly reveal their presence by their pyrotechnic movement, and can easily be picked out.
Some stones will lie table facet down, some pavilion down - it does not matter.

 46: "Red Hot gold - Yellow Hot Moissanite"


Moissanite UK Ltd. were kind enough to provide specimens of their faceted stones for research.  During experiments at Hodgkinsons Ltd., Glasgow,   A colourless moissanite was placed in  molten gold and turned bright yellow, as seen in Fig. 35.

18ct gold melts at  about 900oC, but Mark Kellam at C3 Inc. suggested that, in the two minutes or more that the gold was kept at melting temperature, the moissanite would absorb more heat and was possible reaching twice the temperature of the gold.  Still the stone survived.

At the C3 Inc. modern premises in Morrisville, North Carolina, Dr. Kellam placed a diamond alongside a moissanite in a furnace fed with liquid oxygen.  At about 1050oC, I witnessed the diamond go intensely white hot before it vaporised.  The faceted moissanite survived intact.

The yellow colour transformation of synthetic moissanite under the effect of heat is not thermoluminescence, but simply that the material is yellow at raised temperature.  Intriguingly, the cubic polytype form of silicon carbide is yellow at room temperature, but this isotropic form has only rarely been grown, as by Dr. Nelson in 1948.

On rare occasions a certain type of diamond thermofluoresces under the heating effect of a jewellery workshop gas torch.  In such a rare instance the off-whitish appearance is far removed from moissanite's bright yellow colour change response to heat.  Such rare thermofluorescing diamonds seem always to be associated with a strong 478nm absorption line. Can anyone add confirmation to this, please?

47:  What's Cooking?
After the experiments with the molten gold, I wondered how moissanite's yellow response to heat could be monitored, and at how low a temperature the material would turn yellow.  A mixture of diamonds and moissanites lie cold on the hot plate of an electric oven.  How many moissanites are there?

48: The oven hotplate
The oven hotplate was turned up to a medium setting.  Once hot, it was only seconds before the two moissanites had turned bright yellow.  Such an experiment would allow hundreds of diamonds to be tested at one time.

Glass Filled Diamonds?
Glass fillings leak at such temperatures, so this procedure offers a simple way to check out a diamond parcel.  If sold honestly as "glass filled", then check the diamond identity as described under the heading, "Diamond Parcels". Figs 43, 44.

49: Diamond Scoop
For greatest ease I placed a diamond scoop on the hotplate until it became hot.  Using a glove, I used the scoop to pick up five stones and return them to the oven hot plate.  In seconds you see the four moissanites by their new yellow colour.  Obviously the scoop could handle many stones.
The shiny bright chromium plating of the scoop distracts the eye from seeing the result clearly and I experimented with unpolished aluminium foil on the scoop.  The latter precaution, plus a white overhead light, made the observation instantly positive, especially as the "whiteness" of the diamond provided ample contrast.

50: And so to bed
An ordinary bed light is used here with a 250 watt lamp bulb.  Again the stones turned yellow.  This time at a temperature measured at 231oC.  Lower wattage bulbs did not achieve the colour change.  Dr. Mark Kellam advises that the colour change temperature will have some variation depending on the potential impurities that can be hosted by synthetic moissanite crystals.

51: More than  match for diamond!
The title begs the question.  "Will it work?"  Hold a match (or cigarette lighter) flame tip just below the stone (mounted or loose).  The syntheic  moissanite turns yellow in seconds.  Any sootiness to the stone rinses off in cold water.

52: Moissanite colours
A beautiful  sea green moissanite contrasts with a colourless 1.26ct stone (M colour grade).  In Carolina I was privileged to see a range of beautiful colours, but the main preoccupation was to deliver the colourless goods ordered from many countries around the world.

53:  Bluish Synthetic Moissanite
A more subdued greyish blue synthetic moissanite is reminiscent of certain natural diamond colours
 

Answer to the diamond identity question asked in Figure 1

Top left 1.24ct synthetic moissanite, 1.01ct diamond (F colour), 1.24ct diamond (L colour), 1.45ct strontium titanate, 2.56ct cubic zirconia, 2.22ct GGG, 1.30ct zircon, 1.78ct YAG, 1.11ct synthetic spinel . The stones are arranged in descending refractive order. Because of quality and cutting differences, the two diamonds appear to be two different optical identities.
I would not like to have to answer such a question with the nine stones full frontal in this manner, but hand them to me in a dark room (loose or set), and I will happily separate them one from the other, without seeing them, without touching them, and without instruments. I have had a lot of fun and pleasure sharing this identification technique with gemmologists in various parts of the world, but then "Visual optics" is meant to be fun, apart from its extraordinary contribution to the gem identification programme.

Acknowledgements

Help and encouragement is appreciated from Dr. J Nelson, Doug Morgan, John and Catriona McInnes, Tom Bain, Gillian and the directors of Alan Hodgkinson Ltd, David Wright family, David Pilling family, Richard Cartier, Moissanite UK Ltd., Consultants: Dr. Kurt Nassau and Howard Rubin.   C3 Inc. President Bob Thomas, Chairman  Jeff Hunter, Technical Research Director Dr. Mark Kellam, and Earl Hines, designer and controller of moissanite gem cutting and polishing.