Moissanite: The Second Hardest Mineral

Moissanite is a naturally occurring mineral called silicon carbide, but it is so rare that it is almost never found in nature in large enough pieces to be cut into gemstones. For this reason, all of the moissanite used in jewelry is created in a laboratory. Moissanite was first discovered in a meteor crater in Arizona in 1893.

Moissanite is the name given to naturally occurring silicon carbide and to its various crystalline polymorphs. Silicon carbide is useful for commercial and industrial applications due to its hardness, optical properties and thermal conductivity.

Mineral moissanite was discovered by Henri Moissan while examining rock samples from a meteor crater located in Canyon Diablo, Arizona, in 1893. At first, he mistakenly identified the crystals as diamonds, but in 1904 he identified the crystals as silicon carbide. The mineral was named in his honour.

The mineral form of silicon carbide was named moissanite in honor of Moissan later on in his life. The discovery in the Canyon Diablo meteorite and other places was challenged for a long time as carborundum contamination from man-made abrasive tools.

The colors seen in moissanite from the Mount Carmel area of northern Israel range from dark blue to light green. photo by Aurélien Delaunay.

Natural Moissanite Formation

Natural moissanite, a captivating gemstone with exceptional brilliance, boasts a fascinating genesis rooted in the extreme environments of meteorites and Earth's mantle.

Cosmic Origins:

Extraterrestrial Birthplace: The majority of natural moissanite arrives on Earth within meteorites, celestial visitors formed from the remnants of supernovae or collisions between stars. These interstellar rocks, subjected to immense heat and pressure during their cosmic journeys, crystallize silicon carbide (SiC) into moissanite grains.

Arizona's Pioneering Discovery: In 1893, French scientist Henri Moissan discovered the first terrestrial moissanite within a meteorite crater in Arizona. Initially mistaking these glistening grains for diamonds, he later identified them as a distinct mineral, naming it after himself. This discovery established the connection between meteorites and moissanite formation.

Volcanic Delights: Volcanic eruptions can occasionally bring moissanite to the surface. As molten rock, spewing from Earth's core, cools under specific conditions, rare moissanite formations can crystallize within the solidified magma. The precise formation conditions for Earth-born moissanite remain an active area of research. Scientists believe it requires an environment exceptionally low in oxygen and rich in carbon, coupled with extreme heat and pressure exceeding 1200°C and 30 kilobars.

The colors seen in moissanite from the Mount Carmel area of northern Israel range
from dark blue to light green. Note the broken or rounded morphology.
Composite. photo by Aurélien Delaunay.

Moissanite Properties

Composition: Silicon Carbide (SiC), a naturally occurring mineral formed in meteorites or created in labs under high pressure and temperature.

Color: Typically colorless, but can also be found in shades of yellow, green, brown, and gray.

Luster: Adamantine to metallic, meaning it has a diamond-like brilliance with a hint of metallic sheen.

Crystal System: Hexagonal, featuring six-sided pyramidal crystals and intricate formations.

Streak: Greenish gray, the color powder left behind when scratched.

Hardness: 9.25 on the Mohs scale, second only to diamond (10) and incredibly resistant to scratches and wear.

Cleavage: None, meaning it doesn't break along specific planes like some gemstones.

Crystal Form: Typically found as single crystals or twins, with beautiful geometric shapes.

Density: 3.22 g/cm³, slightly less dense than diamond (3.52 g/cm³).

Transparency: Transparent, allowing light to pass through, enhancing its brilliance.

Fracture: Conchoidal, meaning it breaks with smooth, curved surfaces like shells.

Specific Gravity: 3.22, the ratio of its density to the density of water.

Solubility: Insoluble in common acids and solvents, adding to its durability.

Magnetism: Non-magnetic, not attracted to magnets.

Fluorescence: Weak to moderate fluorescence under long-wave ultraviolet light, appearing blue or green.

Pleochroism: Weak, meaning it shows slight variations in color depending on the viewing angle.

Refractive Index: 2.65 - 2.69, a measure of how much light bends when passing through the stone, contributing to its intense fire.

Inclusions: May contain needle-like inclusions or gas bubbles, especially in natural moissanite, adding a unique character.

Synthetic Moissanite

The creation of Synthetic moissanite can be achieved through several scientific techniques, each offering distinct advantages and control over the final gemstone. Here's a closer look at three main methods:

1. Verneuil Process:

Mechanism: This traditional method utilizes a high-temperature oxyhydrogen flame to melt a mixture of silicon dioxide (SiO₂) and carbon. Tiny droplets of molten material fall onto a seed crystal, gradually building up and solidifying into a moissanite boule.

Advantages: Relatively simple and cost-effective, suitable for creating larger crystals.

Limitations: High potential for inclusion formation, limited control over crystal orientation and color.

2. Czochralski Process:

Mechanism: A crucible containing molten silicon carbide is heated, and a seed crystal is dipped into the melt. As the crystal is slowly pulled upwards and rotated, the molten SiC solidifies onto it, growing a single-crystal moissanite boule.

Advantages: Produces high-quality, inclusion-free crystals with superior optical properties and controlled crystal orientation.

Limitations: Slower growth rate compared to Verneuil, higher equipment cost.

3. Chemical Vapor Deposition (CVD):

Mechanism: Precursor materials like silicon tetrachloride (SiCl₄) and methane (CH₄) are vaporized and introduced into a vacuum chamber. Under carefully controlled temperature, pressure, and gas composition, the vapors react and deposit onto a substrate, forming a layer-by-layer moissanite film.

Advantages: Offers the most precise control over crystal structure, thickness, and dopant incorporation, enabling creation of custom colors and features like rainbow moissanite.

Limitations: Requires sophisticated equipment and high technical expertise, typically produces thinner films compared to other methods.

Conclusion

Synthetic moissanite exhibits superior thermal conductivity compared to diamond, making it less prone to chipping or cracking under sudden temperature changes. Its fire, the rainbow-like play of colors, surpasses that of diamond, adding an extra layer of brilliance to the gemstone.

With its ethical sourcing and controlled creation, man-made moissanite offers a compelling alternative to mined diamonds, attracting environmentally conscious consumers.

Synthetic moissanite boasts a refractive index of 2.65-2.69, exceeding that of diamond (2.42). This translates to an intense fire and brilliance, with rainbows of light dancing within the stone.