Moissanite is a gemstone made of silicon carbide. Its chemical formula is SiC. Diamond is carbon, formula C. They are different materials entirely, in the way that silver and platinum are different metals rather than better and worse versions of one thing.
Where it came from
It was found in 1893 by Henri Moissan, a French chemist, in a meteorite crater in Arizona. He believed he had found diamonds. It took him eleven years to establish that the crystals were silicon carbide instead. He won the Nobel Prize in Chemistry in 1906, for other work.
Natural moissanite is vanishingly rare. It exists in some meteorites and in a few kinds of rock, in grains far too small to cut. Every moissanite ever set into jewellery was grown in a laboratory. This is not a compromise or a secret. There is no other source.
The four numbers that matter
Hardness. Moissanite sits at 9.25 on the Mohs scale. Diamond is 10. That makes it second only to diamond among stones used in jewellery, and it is why moissanite suits daily wear rather than occasional wear.
Refractive index. Moissanite bends light at 2.65 to 2.69. Diamond bends it at 2.42. More light comes back out of a moissanite, so it is the brighter of the two.
Dispersion. This is the splitting of white light into colour, the effect jewellers call fire. Moissanite measures 0.104 against diamond at 0.044. Two and a half times as much.
Specific gravity. Moissanite is 3.22, diamond 3.52. Moissanite is lighter, which has a practical consequence covered below.
Two things run opposite to what people assume
It is more brilliant than diamond, not less. Higher refractive index, and far more dispersion. In ordinary daylight a moissanite throws noticeably more colour than a diamond of the same size. Whether that is what you want is a matter of taste rather than quality, and it is worth knowing before you buy rather than after.
Carat is the wrong measure. Because moissanite weighs less for the same size, a stone sold as one carat is either genuinely one carat and visibly wider than a one carat diamond, or it has been cut to look like a one carat diamond and actually weighs about 0.88. Ask for millimetres. A millimetre measurement describes what you will actually see on your neck or hand.
Why it passes a diamond tester
The common handheld tester does not look at a stone. It touches it with a warmed probe and measures how quickly the heat runs away. Diamond conducts heat better than almost anything on earth, and for decades that was a reliable way to separate it from glass and cubic zirconia.
Silicon carbide happens to conduct heat almost as well. The probe warms it, the heat leaves at a diamond-like rate, and the light goes green.
Testers sold today usually carry a second probe for electrical conductivity. Moissanite conducts electricity and diamond, in almost every case, does not, so a dual tester names moissanite correctly. A jeweller with a loupe needs neither: moissanite is doubly refractive, so looking down through the top facet shows the far edges doubled. Diamond shows them single. It is the oldest test and still the surest.
So moissanite passes the cheap test and fails the proper one. Anyone who tells you it is undetectable is not being straight with you.
On certificates and grades
You will see moissanite advertised with colour and clarity grades borrowed from diamond, and with certificates that resemble diamond reports.
D to Z is the GIA colour scale, built for diamonds. VVS is the GIA clarity scale, also built for diamonds. No independent body assigns either to moissanite. When a listing states a grade, that is the manufacturer describing its own product, not a graded result.
It is not necessarily untrue. Good colourless moissanite really is colourless. But it is self-declared, and stating it as though it were certified is a choice some sellers make and we do not.
Living with it
It will not cloud. Silicon carbide is dense, unporous and chemically settled. It has nothing to absorb and nothing to oxidise. A piece worn daily for years is the same stone at the end.
What dulls is a film on the surface: hand cream, soap rinsed in a hurry, skin oil, hairspray. It gathers underneath the stone, which is exactly where light enters. Warm water, a drop of washing up liquid and a soft brush along the underside restores it.
It will not scratch in normal wear. At 9.25, only diamond and a few laboratory materials can mark it.
The setting is the softer part. Our pieces are moissanite in 925 sterling silver beneath a gold-tone finish. The stone tolerates almost anything; the finish does not. Keep it away from bleach, chlorine, abrasive pastes and ultrasonic cleaners. That caution is about the plating, not the stone, and it is true of any plated jewellery.
So, is it worth buying?
If you want a hard, bright, colourless stone that stands up to being worn every day, moissanite does that at a fraction of the cost of diamond. If you specifically want a diamond, buy a diamond. Moissanite is not a lesser diamond. It is a different stone that happens to be clear and hard, and it is more fiery than the thing it gets compared to.
Our own collection is moissanite necklaces and pendants, each one set in 925 sterling silver beneath a waterproof, tarnish resistant gold tone. What our supplier states about materials we pass on exactly as given. What we have not tested ourselves, we do not present as proven.