Moissanite is silicon carbide, a hard, brilliant crystal that occurs in nature only as tiny grains and is made in laboratories for jewelry. Gem-quality synthetic moissanite has been sold since the late 1990s as a diamond alternative. It is close to diamond in hardness and weight, has more than twice diamond's fire and is doubly refractive, which is the easiest way for a gemologist to tell the two apart.
Henri Moissan and the meteorite crater
The French chemist Henri Moissan, who later won the 1906 Nobel Prize in Chemistry for isolating fluorine, studied rock samples from Canyon Diablo, Arizona, in 1893. The site is the area around Meteor Crater, where fragments of an iron meteorite are scattered across the desert.
In the samples, Moissan found small, hard crystals that he first thought were diamond. In 1904 he identified them as silicon carbide. The natural mineral was named moissanite in his honor in 1905.
Silicon carbide had already been made artificially. In 1891 the American inventor Edward Acheson produced it while trying to synthesize diamond and sold it as an abrasive under the name carborundum. Industrial silicon carbide is still made in large amounts for grinding, cutting and electronics, but it is dark and not gem quality.
Natural moissanite is very rare. It has since been found as microscopic grains in some meteorites, in kimberlite and in a few other rocks, never in pieces large enough to cut as gems.
Synthetic moissanite in jewelry
Growing large, clear silicon carbide crystals took decades of research, mostly for the semiconductor industry. In the 1990s, the company Cree in North Carolina developed a process to grow single crystals suitable for cutting. A partner company, C3 Inc., later renamed Charles and Colvard, brought gem moissanite to market in 1998.
Early moissanite often had a gray, green or yellow tint. Later production improved color, and near-colorless stones became widely available in the 2010s as patents expired and other manufacturers entered the market.
Moissanite is grown by a sublimation process. Silicon carbide powder is heated in a sealed chamber to well over 2,000 degrees Celsius so that it turns to vapor, which then deposits on a cooler seed crystal and builds up a single crystal over time.
Properties compared with diamond
| Property | Diamond | Synthetic moissanite |
|---|---|---|
| Chemistry | Carbon | Silicon carbide |
| Mohs hardness | 10 | 9.25 |
| Refractive index | 2.417, single | 2.65 to 2.69, double |
| Birefringence | None | About 0.043 |
| Dispersion | 0.044 | About 0.104 |
| Specific gravity | 3.52 | 3.20 to 3.22 |
| Thermal conductivity | Very high | High |
| Electrical conductivity | Insulator (except Type IIb) | Usually conducts |
Hardness
At 9.25 on the Mohs hardness scale, moissanite is harder than corundum and much harder than cubic zirconia. It resists scratching well and keeps a sharp polish in everyday wear, which is one reason it became popular for engagement rings.
Fire and brilliance
Moissanite has a higher refractive index than diamond and more than twice its dispersion. Under strong point lights it shows intense rainbow flashes, more than most people expect from a diamond. Some buyers like that look, while others find it reads as not quite diamond. The diamond optical properties page explains how dispersion produces fire.
Weight
Moissanite is slightly lighter than diamond. A moissanite with the same dimensions as a 1-carat diamond weighs a little less, around 0.88 to 0.9 carat. Sellers usually describe moissanite by millimeter size or diamond-equivalent weight.
Double refraction
The clearest optical difference is double refraction. Diamond is cubic and singly refractive. Gem moissanite belongs to hexagonal forms of silicon carbide (most often the 6H and 4H polytypes), so light passing through most directions splits into two rays traveling at different speeds.
Looking through the crown with a 10x loupe at an angle, a gemologist sees the pavilion facet junctions doubled, two lines where diamond shows one. Cutters often orient moissanite with the optic axis running perpendicular to the table, so the doubling does not show when looking straight down. Tilting the stone and viewing through the side facets reveals it. The birefringence page covers double refraction in other gems.
Other clues under magnification include needle-like inclusions running parallel to each other, a feature of the growth process, and a girdle that is usually polished instead of frosted.
The thermal tester problem
For about two decades before moissanite arrived, the standard jeweler's diamond tester measured thermal conductivity. Diamond conducts heat far better than any other gem, so a probe that sensed rapid heat flow could separate diamond from cubic zirconia, glass, white sapphire and other simulants.
Moissanite also conducts heat well, close enough that thermal testers read it as diamond. When moissanite reached the market in the late 1990s, this created real concern in the trade, since a test jewelers had relied on for years could now give a false positive.
Electrical conductivity testing
Testers built to catch moissanite rely on electrical conductivity. Most gem moissanite conducts a small electrical current, while nearly all diamonds are electrical insulators. Combined testers measure heat flow first and then electrical conductivity, flagging a stone that passes the thermal test and also conducts electricity as moissanite.
There is one exception. Natural blue Type IIb diamonds contain boron and also conduct electricity, and some lab-grown diamonds grown with boron behave the same way. A conductivity tester can flag these real diamonds as moissanite, so a blue or grayish stone needs further testing. The diamond chemistry and formation page explains Type IIb diamonds.
Moissanite versus lab-grown diamond
Moissanite is a simulant. It imitates diamond but is a different material. A lab-grown diamond is diamond, with identical chemistry and properties. Both are made in labs, which causes confusion, but they look and test differently. Moissanite has more fire and shows doubling, while lab-grown diamond can only be separated from natural diamond with laboratory instruments. In the United States, the FTC's Jewelry Guides treat both terms as needing clear disclosure, and moissanite must not be described as diamond.
Care
Moissanite is durable and stable. It tolerates heat better than most gems, resists most chemicals and can be cleaned with warm soapy water or, in most settings, an ultrasonic cleaner. The jewelry cleaning guide covers safe methods for different gems.
Frequently asked questions
Is moissanite a diamond?
No. Moissanite is silicon carbide, a different material. It is sold as a diamond alternative and must be disclosed as moissanite.
Will moissanite pass a diamond tester?
It passes most thermal-only testers, which read it as diamond. Testers that also measure electrical conductivity identify it as moissanite.
Does moissanite have more sparkle than diamond?
It shows more fire, meaning colored flashes, because its dispersion is more than twice diamond's. Some people prefer this and others find it less like diamond.
Where does natural moissanite come from?
Natural moissanite occurs as microscopic grains in some meteorites and a few rocks. Henri Moissan first identified it in material from Canyon Diablo, Arizona. All jewelry moissanite is synthetic.