How to Tell If a Diamond Is Real: 9 Tests You Can Trust

Most "is my diamond real?" advice on the internet is half right — which is dangerous, because the half that's wrong is usually the part that lets a moissanite slip through. Here's what actually works, ranked by how much you can trust it.

Fast at-home screens

1. The fog test

Breathe onto the stone. A real diamond disperses heat so efficiently that the fog clears almost instantly. Glass and cubic zirconia stay misty for two to four seconds. Caveat: moissanite also clears fast, so a pass here is necessary, not sufficient.

2. The read-through test

Place a loose stone table-down on a line of newsprint. A well-cut diamond bends light so sharply you can't read the letters through it. If the text is legible, you're likely looking at glass or quartz.

3. Sparkle: brilliance vs. fire

Diamond returns a crisp mix of grey-white brilliance and controlled rainbow fire. Moissanite throws so much fire it can look almost like a disco ball. Once you've seen the difference side by side, it's hard to unsee.

4. The water test

Drop the stone in a glass of water. A real diamond's high density (3.52 g/cm³) sends it straight to the bottom. A floating or slow-sinking stone is a red flag — though a mounted stone makes this unreliable.

Tests that need a tool

5. Thermal conductivity pen

The classic "diamond tester." It works — but only against CZ and glass. Moissanite conducts heat similarly to diamond and passes these pens routinely. This single blind spot is why so many people are fooled.

6. Electrical conductivity stage

This is the upgrade that catches moissanite. Diamond is an electrical insulator; moissanite conducts. A combined thermal + electrical tester separates the three most common stones in seconds.

7. Loupe inspection

Under 10x magnification, look at the girdle and facet junctions. Natural diamonds often show tiny inclusions and sharp facet edges. Simulants tend to look too clean, with slightly rounded facet meets.

What only a lab can settle

8. Refractive index & dispersion

Instruments measure exactly how light slows and splits inside the stone. Diamond sits at 2.42; moissanite at 2.65; CZ at 2.15. There's no faking these numbers.

9. Spectroscopy (natural vs. lab-grown)

A real diamond can still be lab-grown — chemically identical, but worth far less. Only spectroscopy and fluorescence patterns reliably separate earth-mined from lab-created. If you're insuring or reselling, this is the test that matters.

The honest bottom line

At-home tests are great for ruling out obvious fakes. They are not reliable for separating diamond from moissanite, or natural from lab-grown — the two distinctions that actually move value. When money or insurance is on the line, get an instrument test. That's exactly what Diamonds Tester's photo screen and lab certification are built for.