Moissanite has a higher refractive index and more than double the dispersion of diamond. That's the physics behind why it looks the way it does: more fire, comparable brilliance, and a different visual character depending on the light it's in.
Refractive index: why moissanite bends more light
Refractive index measures how much a material bends light as it passes through, and the more it bends, the more light comes back out to your eye as brilliance. Moissanite's refractive index is 2.65. Diamond's is 2.42. Sapphire and ruby sit at 1.77, emerald at 1.58. That gap between moissanite and diamond isn't marginal, it's nearly 10 percent more light bending, which is why a well-cut moissanite returns so much light.
Fire: the rainbow effect
Dispersion is what splits white light into spectral colour as it exits a stone, and moissanite's dispersion rate of 0.104 is more than double diamond's 0.044. In direct sunlight or under a strong point light source, that difference is obvious: moissanite throws visibly more coloured flashes than diamond does. Some people love the intensity. Others prefer diamond's more restrained white sparkle. Neither preference is wrong, and it's worth seeing both in person before you decide, which is exactly what a design consultation is for.
Brilliance: moissanite doesn't give this up for fire
Brilliance is the white light return, separate from fire. A well-cut round brilliant moissanite produces excellent white light return precisely because its high refractive index keeps more light inside the stone before it exits through the top. Moissanite's optical character isn't inferior to diamond's, it's different: more fire, comparable brilliance, and the difference is most visible in direct sunlight. Indoors, under mixed ambient lighting, most people can't reliably tell the two apart on optical character alone.
How shape changes the effect
Round brilliant produces the most fire and brilliance of any shape, since it's cut specifically to maximise light return from every angle. Oval carries high brilliance with fire spread more softly across its longer outline. Cushion varies by faceting style, crushed ice shows strong brilliance, chunkier antique faceting softens the fire. Pear combines round's brilliance at the rounded end with a distinct light play at the point. Emerald is the outlier: its step-cut facets produce broad flashes of white light instead of broken spectral fire, dramatic rather than sparkling.
Moissanite vs diamond, side by side
Diamond has been the benchmark gemstone for centuries for good reason, its optical performance is genuinely excellent. Moissanite doesn't copy that performance, it exceeds it on the measurable numbers (refractive index and dispersion) while producing a visually distinct result some buyers prefer and others don't. The difference shows up most in direct sunlight; indoors, it's often not something most observers would notice unprompted. Our full moissanite vs diamond comparison covers price, hardness, and certification alongside the optics.
Frequently asked questions
Does moissanite sparkle more than diamond?
It produces more fire, more than double diamond's dispersion, and comparable brilliance. Whether that adds up to "more sparkle" depends on which effect you mean.
What is moissanite's refractive index?
2.65, against diamond's 2.42, and higher than any other commonly used gemstone.
Why does moissanite show rainbow colours?
Its dispersion rate of 0.104 is more than double diamond's, which splits more white light into spectral colour as it exits the stone.
Which moissanite shape has the most fire?
Round brilliant, by a clear margin. Emerald and other step cuts show the least, trading fire for broad, dramatic flashes of white light instead.
See the difference for yourself across our moissanite engagement ring collection, read the full What Is Moissanite guide for the complete picture, or start a free design consultation and we'll help you choose the shape and cut that suits the look you actually want.