For a century, cubic zirconia was the diamond simulant to beat. Then moissanite arrived, harder, more brilliant, and — crucially — able to fool the thermal testers that jewellers had relied on to catch CZ. Both are sold honestly as what they are and dishonestly as diamond. A laboratory separates all three with a short sequence of tests, none of which harms the stone.
The three materials
| Diamond | Moissanite | Cubic zirconia | |
|---|---|---|---|
| Composition | Carbon | Silicon carbide | Zirconium oxide |
| Refractive index | 2.417 | 2.65–2.69 | 2.15–2.18 |
| Double refraction | None | Yes (0.043) | None |
| Dispersion ("fire") | 0.044 | 0.104 | 0.060 |
| Hardness (Mohs) | 10 | 9.25 | 8–8.5 |
| Specific gravity | 3.52 | 3.22 | 5.6–6.0 |
| Thermal conductivity | Very high | High | Low |
| Electrical conductivity | Insulator (most) | Conductor (most) | Insulator |
The thermal tester
The hand-held "diamond tester" measures how quickly a probe tip loses heat into the stone. Diamond conducts heat extremely well and the tester beeps; CZ, glass and most other simulants do not, and it stays quiet. This is a good separation of diamond from CZ.
It fails with moissanite, which also conducts heat well and beeps as diamond. A thermal tester that says "diamond" has said "diamond or moissanite".
The electrical tester
Most moissanite conducts electricity; most diamond does not. Combined thermal-electrical testers use both properties and separate all three: CZ fails thermal; moissanite passes thermal but conducts electricity; diamond passes thermal and does not conduct. This is the standard quick screen in the trade. Its limits: some natural blue diamonds and some laboratory-grown diamonds conduct electricity, and the instrument then needs backing up.
Doubling
Moissanite is doubly refractive; diamond and CZ are not. Looking through a moissanite at 10× toward the far facet junctions, the edges appear doubled — two lines instead of one — in most viewing directions (not down the optic axis, so the examiner tilts the stone). Diamond and CZ show single, crisp edges. This is the most reliable loupe test for moissanite.
Dispersion and appearance
Moissanite's dispersion is more than twice diamond's, giving it noticeably more coloured flashes — "too much fire", to an experienced eye, especially in larger stones under spot lighting. CZ is also more dispersive than diamond and tends to look slightly "glassy" with less contrast between light and dark areas. These are impressions, useful for suspicion, not conclusions.
Weight and size
CZ is more than half again as dense as diamond. A stone of 6.5 mm diameter — a one-carat round diamond — weighs about 1.7 ct if it is CZ. Moissanite is slightly lighter than diamond. A quick weight-versus-size check with a gauge and a balance separates CZ at once.
Under the microscope
Diamond shows natural inclusions — crystals, feathers, pinpoints — and, on the girdle, may show natural surface features. CZ is usually flawless or shows gas bubbles. Moissanite frequently shows fine, parallel needle-like inclusions and may have a slight greyish or greenish tint, and its facet junctions are sometimes rounded because it is cut differently. Diamond's extreme hardness gives it sharper facet edges than either simulant.
Refractive index
All three read "over the limit" on the standard refractometer. Reflectivity meters, which infer RI from surface reflection, separate them, but the tests above are usually enough.
The other simulants
Glass, white sapphire, white topaz, white zircon, YAG, GGG and strontium titanate have all been sold as diamond. None conducts heat like diamond; all fail the thermal tester; zircon and strontium titanate show doubling or extreme dispersion; RI and SG separate the rest. How a gemmological laboratory tests a gemstone.
Laboratory-grown diamond is diamond
Note that a laboratory-grown diamond passes every test above as diamond, because it is diamond. Separating it from natural diamond is a different problem, addressed in Lab-grown diamonds (HPHT and CVD).
Frequently asked questions
Will a diamond tester detect moissanite?
A thermal-only tester will not; it reads moissanite as diamond. A combined thermal-electrical tester will, in most cases.
Can I see doubling without a microscope?
With a 10× loupe and practice, yes, in moissanite over a few millimetres. Look at the far facet edges through the table while tilting the stone.
Is moissanite a fake diamond?
It is a distinct material, sold legitimately under its own name. It is a fake only when sold as diamond.
Which is harder to detect, moissanite or CZ?
Moissanite, because it passes the thermal tester and is harder and more brilliant. CZ is caught by the thermal tester and by its weight.
Does a diamond certificate guarantee it is not moissanite?
A report from a laboratory that examined the stone, with weight and dimensions that match, is strong evidence. A certificate with no matching details could accompany any stone.