Moissanite vs Diamond: Fire, Hardness, Testers and Value
The short answer
Moissanite is a silicon carbide gemstone that closely mimics diamond but differs in several key optical and physical ways. Moissanite has higher dispersion and fire, producing rainbow flashes, and exhibits double refraction (birefringence) visible under 10x magnification. While standard thermal testers may confuse moissanite with diamond, electrical conductivity testers and gemological loupes easily tell them apart.
Origin and Composition: Silicon Carbide vs Pure Carbon
Natural moissanite is extraordinarily rare, first discovered in 1893 by French chemist Henri Moissan inside a meteor crater in Arizona. Virtually all moissanite used in jewellery today is synthesised in laboratories from silicon carbide (SiC).
Diamonds, by contrast, are composed of pure crystalline carbon arranged in an isometric cubic crystal lattice. Natural diamonds formed billions of years ago in Earth's mantle, while lab-grown diamonds share the exact identical carbon structure.
Fire and Brilliance: The Rainbow Disco-Ball Effect
The easiest visual giveaway between moissanite and diamond is their optical dispersion:
- Diamond: Refractive index of 2.417 with a dispersion score of 0.044. It emits clean, crisp white flashes (brilliance) combined with subtle, balanced spectral fire.
- Moissanite: Refractive index of 2.654 to 2.691 and a dispersion score of 0.104—more than double that of diamond. In direct sunlight, moissanite produces intense, colourful rainbow flashes often described as the 'disco-ball effect'.
Double Refraction (Birefringence) Under a Loupe
Diamond is singly refractive (isotropic): light entering the stone travels at a single velocity and exits without splitting. Moissanite is doubly refractive (birefringent).
When examining a moissanite through the side or crown facets using a 10x jeweller's loupe, you will observe facet doubling: the back pavilion facet edges appear doubled or blurred. Diamond facet edges remain razor-sharp from every single viewing angle.
Hardness and Durability: Mohs 9.25 vs 10
Diamond is the hardest known natural substance, rating a 10 on the Mohs scale. Moissanite rates 9.25, making it the second-hardest gemstone used in jewellery.
While 9.25 is exceptionally durable and entirely suitable for daily lifetime wear (far superior to cubic zirconia at 8.0 or quartz at 7.0), diamond is still approximately four times harder in absolute scratch resistance. Read our guide on diamond 4Cs for more details on diamond grading.
Why Cheap Diamond Testers Get Fooled
Basic diamond testers measure thermal conductivity. Because moissanite conducts heat remarkably well—almost identical to diamond—basic thermal tester pens will beep positive for 'diamond'.
Professional jewellers use multi-testers that measure both thermal and electrical conductivity. Diamond is an electrical insulator, whereas moissanite conducts electricity. A dual tester detects this difference instantly.
Pricing and Resale Value
Moissanite costs roughly 5% to 10% of the price of an equivalent natural diamond. However, moissanite holds virtually zero secondary resale value—pawnbrokers and gold buyers value only the metal setting. Learn how secondary markets price fine pieces in what is my jewellery worth.
Common questions
Can a jeweller tell moissanite from diamond immediately?
Yes. Under a 10x magnification loupe, a jeweller instantly checks for facet doubling caused by double refraction, as well as the intense rainbow dispersion.
Will moissanite pass a standard diamond tester?
It will pass a thermal tester because moissanite conducts heat similarly to diamond. It will not pass an electrical multi-tester.
Does moissanite get cloudy or lose sparkle over time?
No. Moissanite has a hardness of 9.25 and does not cloud, fade, or degrade over time. Surface oils can temporarily dull sparkle, but a standard cleaning restores full brilliance.
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