Moissanite: diamond imitationBy

Moissanite: diamond imitation

passes as genuine diamond on thermal testers

In an experiment conducted in late 1997, 40 jewelers and pawn shop owners incorrectly identified synthetic moissanite, a new diamond substitute, as genuine diamond. They were fooled because they all used what was until then the most widely used diamond test, a thermal conductivity probe.

Thermal probes, until recently called simply "diamond testers", separate imitations from real diamond by detecting or measuring a stone's ability to transmit or transfer heat. Synthetic moissanite's thermal properties are so close to those of diamond, however, that thermal diamond testers register synthetic moissanite as genuine diamond.

Although synthetic moissanite is manufactured and distributed by a highly reputable company, many fear synthetic moissanite will increasingly be represented by unethical sellers as diamonds. There is also a great potential for using synthetic moissanite in repair and replacement settings in antique and collectible jewelry.

This article will examine what moissanite is, explain tests to identify and separate synthetic moissanite from diamond and describe new test equipment designed specifically to detect synthetic moissanite. And keep your thermal probe. Thermal probes are an essential part of synthetic moissanite identification.

What is moissanite?

Moissanite is silicon carbide (SiC). Naturally occurring moissanite is very rare. It is present in only trace amounts on earth and has been found on meteorites. In the late 1890s, Dr. Henri Moissan a French scientist discovered natural silicon carbide in a meteor in Arizona. The mineral name for silicon carbide, moissanite, is taken from his name.

Synthetic silicon carbide has been produced in laboratories since 1893. One of the early experts was Edward Acheson who developed the material as an abrasive which he called carborundum. It is still used in sandpaper, grinding wheels and other industrial applications. Manufactured bulk silicon carbide is one of the most common materials: 69,900 metric tons were produced in the U.S. in 1998, 223,000 more tons were imported. (Balazik, U.S. Geologic Survey, 1999).

Since the late 1940s, research has focused on developing large single crystals of synthetic silicon carbide (moissanite). Such crystals were sought after for use as semiconductors in the electronics industry and, it was always hoped, imitation diamonds. Gem quality colored moissanite was not produced until 1990. Colorless synthetic moissanite suitable as a diamond substitute was not produced in commercial quantities until late 1997 and not widely marketed until 1998.

Jewelry grade synthetic moissanite was manufactured in the United States by Cree Research, Inc. which initially developed silicon carbide products for the electronics industry. The Cree crystals were sold to and marketed by C3, Inc. which cuts and polishes the crystals into finished products. Synthetic moissanite is being positioned to compete with cubic zirconium which has sales of $200 million a year.

By October, 1998, C3 Inc. was distributing synthetic moissanite in the United States through 52 retail locations in 47 cities. Production was estimated at 1500 carats in July 1998 and 2000 carats in August 1998 (Jones, 1998). Synthetic moissanite is also being promoted through various Internet sites.

Prices of synthetic moissanite are about 5-10% that of diamonds. For this article, ACRN paid $122 for a loose 5mm round brilliant cut sample of synthetic moissanite. That size is roughly equivalent to about a one-half carat diamond. A good quality loose diamond that size and cut, depending on specific color and clarity, would have retailed for about $1200-$2000 around the same time period.

Automatic moissanite testers

The easiest way for the general dealer, auctioneer or appraiser to identify synthetic moissanite is with an automatic handheld tester specifically designed for that purpose. A variety of new equipment has been developed to test for synthetic moissanite. Standard diamond testers, which measure thermal conductivity, will not detect moissanite; moissanite passes as genuine diamond on thermal diamond testers.

Moissanite testers measure electrical conductivity, not thermal conductivity. Various catalysts added to the synthetic moissanite crystals to enhance growth increase electrical conductivity. Electrical conductivity of natural diamond is much lower. Moissanite testers detect the difference in electrical conductivity and signal which material, diamond or moissanite, is being analyzed.

The other technique currently used in moissanite testers is based on ultraviolet light. Synthetic moissanite absorbs ultraviolet, diamond transmits or passes ultraviolet. Testers based on ultraviolet currently cost about twice the price of testers that measure electrical conductivity.

Thermal testers, though, are still needed to distinguish between cubic zirconia, glass, paste and other types of imitations. But since synthetic moissanite tests as diamond on thermal testers, a thermal test alone is not conclusive. Any stone which passes the thermal test must now be tested a second time with a specialized moissanite tester. In other words, you need both a thermal tester and a specialized moissanite tester. Both tests are now included in one handheld unit. The thermal test is performed first immediately followed by a test of electrical conductivity. These testers sell for about $225 and up.

For best results, all test stones and probes must be clean and dry for accurate results. To be safe, test on more than one spot on each stone.

Traditional testing

The alternative to detecting synthetic moissanite with automatic test equipment is to conduct a traditional gemological inspection. This requires specialized equipment like a polariscope, binocular microscope, specific gravity liquids, etc. Unless you are a jeweler, you're not likely to own such items and it would be less expensive to just buy the automatic testers. For those readers with the equipment and skill to conduct gemological tests, differences in the physical properties between diamond and synthetic moissanite are listed in the chart in Fig. 7.

Visual tests

Some visual tests may be attempted with a good quality 10X loupe. Synthetic moissanite, for example, is doubly refractive; diamond is singly refractive. Depending on orientation, the doubling of facets may be seen with a 10X hand held loupe. Try to focus on the culet while looking through the main crown facets. Ability to see the doubling seems directly related to one's experience with gems. Persons experienced looking at gems are usually able to detect the doubling; those with little or no experience generally have difficulty detecting the doubling. When using a hand held loupe, doubling is usually, but not always, easier to see on larger stones.

Many facet junctions tend to be rounded in cut synthetic moissanite. Facet junctions in cut diamonds are sharp and straight. Synthetic moissanite also commonly has white needlelike inclusions. Depending on the size of the stone, the facet junctions and needled inclusions are difficult if not impossible to see much below 15-20X magnification.

Many pieces of synthetic moissanite tend to display ("flash") a rainbow or prism-like display of color. Genuine colorless diamonds display fewer colors.

Summary

Diamond testers based on thermal conductivity alone are no longer reliable indicators of genuine diamonds. Special synthetic moissanite testers must now be used to establish a diamond's authenticity. Although the vast majority of synthetic moissanite is currently sold mounted in jewelry, be alert for it in replacements and repairs to antique pieces.

There is little confusion between synthetic moissanite and diamond when subjected to standard gemological testing under a jeweler's microscope or polariscope. Visual inspection with a hand held loupe, however, is not reliable and is probably beyond the skills of the average general line antiques dealer or appraiser.

Editor's Note: C3, Inc., manufacturer of synthetic moissanite, has provided a continuous flow of technical information on its products to the jewelry trade and general public. C3, Inc. routinely makes samples available for independent and third party testing to help reduce and eliminate confusion between synthetic moissanite and diamonds.

Balazik, R. U.S. Geological Survey, Mineral Industry Surveys, Manufactured Abrasives in the Fourth Quarter 1998, March 1999, Reston, VA.

FDJ Tools, on-line bulletin Diamond or Moissanite, January 1999, Winter Park, FL.

Grampp, R.B., Nubert S., Synthetic Moissanite Test Machine Update, Bluestone Trading, Cleveland, OH. 1999. more info. 1-888-800-8508 (this number looks strange but it is correct, ED).

Jones, F. Beware of Synthetic Moissanite. Orchid List and Digest, Electronic Forum for Jewelry Mfrs., Oct. 1998.

Nassau, K., McClure, S., Elen, S., Shigley, J. Synthetic Moissanite: A New Diamond Substitute, Gems & Gemology, Winter Vol. XXXIII, Winter 1997, Gemological Institute of America, Carlsbad, CA.

Image

Fig. 1 Synthetic moissanite by C3, Inc. Photo: C3, Inc.

Parts of a stone

Table: also called the face, the table is the flat top of the stone.

Crown: the upper portion of the stone, the part of the stone above the girdle.

Girdle: the dividing line between the upper and lower portion of the stone. Forms an edge making up the widest border of the stone. The girdle is usually gripped by prongs or bezel when mounted.

Pavilion: the lower portion of the stone, the part of the stone below the girdle.

Culet: the lowest point of the stone, the bottom of the pavilion.

Facet: flat faces or planes that have been cut and polished

Shape: refers the basic outline of the stone: round, rectangle, octagon, oval, teardrop, antique.

Cut: how a finished diamond is created from a rough stone. Cutting and shape are not the same.

Image

Fig. 2

Image

Fig. 3 Selection of near colorless synthetic moissanite in round brilliant cuts from C3, Inc. Actual sizes from 4mm (.2 carats) to 10mm (3.17 carats). Average retail prices range from $72.50 for 4mm to $3,000+ for 10mm. Photo: C3, Inc.

Image

Fig. 4 Synthetic near colorless moissanite, 5mm (approx. ½ carat), round brilliant cut ACRN purchased for $122. It passes as diamond on thermal diamond tester.

Image

Fig. 5 Faceted synthetic moissanite magnified 10X. Doubling of facets as seen through crown. Synthetic moissanite is doubly refractive; natural diamond is singly refractive.

Image

Fig. 6 A polariscope tests optical properties of stones such as whether a stone is singly or doubly refractive. Synthetic moissanite (doubly refractive) should be examined in several directions for accurate readings. Polariscopes retail for $200-$300. Model shown is by Gemological Products. Price $225 from Kessler Sales.

Properties of Colorless diamond vs. colorless simulants

Mohs hardness Specific gravity Refractive index Optic property
Diamond 10 3.52 2.417 Singly refractive
Synthetic moissanite 3.22 2.648,2.691 Doubly refractive
Cubic zirconia 8-8½ 5.56-6.00 2.150-2.180 Singly refractive

Fig. 7 Chart compares properties of colorless diamond to colorless simulants, synthetic moissanite and cubic zirconia.
Source: Gems & Gemology, 1997, Carlsbad, CA.

How to use diamond-moissanite test equipment

Image

Fig. 8

Image

Fig. 9 Facet junctions of synthetic moissanite tend to be rounded. Facet junctions in diamonds are sharp and flat.