How to tell if a diamond is real
Home tests narrow it down. They don’t confirm it.
By the CTJ Jewellery team, led by founder Greg Ward · Updated September 2026 · Editorial policy
Key facts
- Home tests rule out glass, not moissanite
- Lab-grown diamonds are real diamonds
- Natural versus lab-grown needs laboratory equipment
- Check the laser inscription against the report
- Verify GIA and IGI reports online
- A registered valuer can confirm a stone
Do home diamond tests work?
Some help rule out glass or cubic zirconia. None can confirm a diamond on its own.
| Test | How it works | What it tells you |
|---|---|---|
| Fog test | Breathe on the stone | Diamond clears almost instantly. So does moissanite. |
| Water test | Drop a loose stone in water | Diamond sinks, but so do most simulants. Unreliable. |
| Read-through test | Place a loose stone on print | A well-cut diamond blurs the print. Many simulants do too. |
| Loupe | Look at 10× magnification | Crisp facet edges and natural inclusions suggest diamond. Not proof. |
| Thermal tester | Probe the stone | Separates diamond from glass and CZ. Most moissanite reads as diamond. |
How do you tell a diamond from moissanite?
Moissanite shows strong rainbow flashes and double refraction: under a loupe, facet edges look doubled. Basic thermal testers can’t reliably tell the two apart, so use a combined diamond and moissanite tester or ask a professional. See lab-grown diamonds and moissanite compared.
Can you tell a natural diamond from a lab-grown one?
Not by eye, loupe or home tester. Both are diamond. Laboratories use spectroscopy and screening instruments to find differences in how each was formed. Reputable grading reports state the origin, and GIA and IGI laser inscribe the lab-grown diamonds they grade with a lab-grown mark and report number.
How do I check a grading report?
- Enter the report number on the GIA or IGI website
- Check the carat weight, measurements and grades match
- Find the matching laser inscription under a loupe
- Be wary of reports from laboratories you can’t verify
More on grading reports.
Where can I get a diamond checked?
A registered valuer or a gemmological laboratory can confirm what a stone is. CTJ arranges independent diamond valuations through a registered valuer we work with.