First, what a “fake” diamond usually is
Most stones mistaken for diamonds are one of three things: cubic zirconia (a cheap, common simulant), moissanite (a harder, more convincing simulant), or glass and crystal. These are not diamonds at all — they only imitate the look.
This matters because the tests below rule out weak imitations easily, but moissanite in particular can pass several of them. Keep that in mind before drawing a firm conclusion.
The fog test
Breathe onto the stone as you would a mirror. A real diamond disperses heat almost instantly, so the fog clears in a moment. A glass or cubic zirconia imitation holds the fog noticeably longer.
It is a useful first filter, not proof — moissanite also sheds heat quickly and can clear as fast as a diamond.
The water test
Drop a loose stone into a glass of water. A diamond's high density (around 3.5) sends it straight to the bottom. Many hollow-backed glass fakes sink slowly or drift.
Treat this as a rough indicator only: several simulants are dense enough to sink as well.
The read-through (newspaper) test
Place a loose, clean stone flat-side down over a line of printed text. A well-cut round-brilliant diamond bends light so sharply that you cannot read the letters through it.
If you can read the print clearly, the stone is likely glass or quartz. This works best on round brilliants; step-cut and fancy shapes refract differently, so a clear read is not a reliable verdict for those.
Sparkle, fire and the loupe
Held to the light, a diamond returns crisp white light (brilliance) with restrained flashes of colour (fire). Cubic zirconia tends to throw excessive rainbow flashes; a tired, greyish sparkle can suggest glass.
Under a 10× loupe, look for tiny natural inclusions — a diamond is rarely flawless. Under the same loupe, moissanite often shows a faint doubling of facet edges (birefringence) that a diamond does not.
- Too much rainbow, too little white light — think cubic zirconia.
- Doubled facet edges under a loupe — think moissanite.
- A perfectly clean, characterless interior — be suspicious.
Check the setting
Jewellers rarely set a genuine, valuable diamond in cheap metal. Look inside the band for hallmarks: 10K, 14K, 18K, 585, 750, PT or 950 point to gold or platinum and a stone worth setting well.
A stamp reading CZ, or base-metal plating that is wearing through, is a quiet warning.
Where home tests end — and verification begins
A thermal diamond tester is a step up, but standard models mistake moissanite for diamond because both conduct heat well; only a combined thermal-and-electrical tester separates the two reliably.
For anything you intend to insure, buy or sell, the honest answer is that certainty comes from a gemological laboratory or a documented record — magnified imaging, measured properties and a considered reading. Our Diamond Value Estimator can give you an indicative figure in the meantime, but it is a guide, never a guarantee.