If It Scratches Glass, It’s a Diamond — Common Tests That Don’t Work

Many old diamond tests like scratching glass or fogging the stone don’t prove authenticity. Learn why these methods fail and safer ways to test your diamond.

You’ve probably heard someone say, “If it scratches glass, it’s a diamond.”

That’s one of the most repeated myths in the jewelry world. I’ve explained this countless times to buyers, sellers, and collectors.

Let’s break down the most common “tests” and why they don’t work.

1. Scratching Glass

• Diamonds are hard, but so are other stones like quartz or moissanite.
• Many gems can scratch glass. It doesn’t make them diamonds.
• You risk damaging your stone if you try this.

Have you ever scratched a window with a key? That didn’t make your key a diamond.

2. The Fog Test

• People say if you breathe on a diamond, it shouldn’t fog up.
• In reality, almost any polished stone clears quickly.
• Temperature and humidity also affect this test.

Why risk relying on breath when gem labs use advanced tools?

3. The Lighter or Heat Test

• Some believe heating a stone with a lighter proves if it’s real.
• Exposing gems to sudden heat can cause cracks or fractures.
• Lab-grown diamonds and natural diamonds react the same way.

Would you burn money to see if it’s real? Then don’t burn a diamond.

4. Sparkle or “Rainbow” Test

• People often judge by sparkle or rainbow flashes.
• Cubic zirconia and moissanite sparkle even more than diamonds.
• Lighting conditions change everything.

If you’ve ever seen a disco ball, you know sparkle alone proves nothing.

5. Weight Check

• Some try to guess authenticity by holding the stone.
• Cubic zirconia is heavier than diamond.
• But weight alone can’t confirm what the stone is.

Can you really tell a few grams difference by hand?

The Better Way

If you want to know if your stone is a diamond:

• Use a professional diamond tester.
• Ask a certified gemologist.
• Request a lab report from GIA, IGI, or HRD.

That’s how you get answers you can trust.

This post debunks popular diamond myths. It explains why scratching glass, fogging the stone, using a lighter, or checking sparkle are unreliable tests. Readers will discover practical, safe alternatives for identifying real diamonds.