The biggest difference between diamond testers is whether they can catch moissanite. Basic thermal testers can tell a diamond from glass, CZ and most simulants, but moissanite moves heat the way a diamond does, so it reads as a diamond. If you buy or take in stones you did not set yourself, you need a tester that separates diamonds from moissanite.

Diamond Testers Compared

	Tester / Segregator	Diamonds vs CZ and glass	Catches moissanite	Colored gemstones	Natural vs lab-grown
	Diamond Selector II	Yes	No	No	No
	Presidium Gem Tester PGT II	Yes	No	Yes	No
	Presidium Duo Tester PDT II	Yes	Yes	Yes	No
	Gemlogis Astro Two-way	Yes	Yes	Yes	No
	Gemlogis Belize	Yes	Yes	No	Yes
	Gemlogis Belize PRO	Yes	Yes	With Astro	Yes

How Diamond Testers Work

Thermal testers touch the stone with a heated probe and measure how fast heat moves through it. A diamond moves heat much faster than CZ, glass or most simulants, so it reads differently. Moissanite moves heat almost exactly like a diamond, which is why a thermal-only tester cannot tell the two apart.

Thermal plus reflectivity or electrical testers add a second check. Moissanite conducts electricity and reflects light differently than a diamond, so a second reading separates them. These are the testers to use when you are buying or taking in stones.

The Testers and Segregators

Diamond Selector II

A simple thermal tester that separates diamonds from CZ, glass and most simulants. Good as a first tester or a backup at the bench. It does not distinguish moissanite.

Shop Diamond Selector II

Presidium Gem Tester PGT II

A thermal tester that also helps identify common colored gemstones like ruby, sapphire and emerald. Diamonds and moissanite read in the same group, so pair it with a moissanite tester if you buy stones.

Shop PGT II

Presidium Duo Tester PDT II

Combines a thermal test with a reflectivity meter, so it separates diamonds from moissanite and helps identify colored gemstones. The all-around choice for buying, trade-ins and pawn.

Shop Duo Tester PDT II

Gemlogis Astro Two-way Gem Tester

Separates diamonds from moissanite and identifies up to 29 types of gemstones on a large touch screen. Works on its own with a wired probe, or wirelessly with the Belize PRO.

Shop Gemlogis Astro

Gemlogis Belize

A portable diamond segregator that separates natural diamonds from lab-grown (CVD and HPHT) diamonds, simulants and moissanite, in loose or mounted stones.

Shop Gemlogis Belize

Gemlogis Belize PRO

An all-in-one diamond segregator that separates natural diamonds from lab-grown diamonds and flags moissanite, down to 0.02 ct. Pair it with the Astro to add colored gemstone readings. Also available as a bundle.

Shop Belize PRO Belize PRO + Astro Bundle

What About Lab-Grown Diamonds?

Lab-grown diamonds are real diamonds, so standard diamond testers read them as diamonds. Separating natural from lab-grown takes a segregator built for it, like the Gemlogis Belize, the Belize PRO, or the Presidium Synthetic Diamond Screener II (SDS II).

Frequently Asked Questions

Can a diamond tester detect moissanite?

Only some can. Thermal-only testers read moissanite as a diamond. You need a tester with a second check, such as reflectivity or electrical conductivity, like the Presidium Duo Tester PDT II, Gemlogis Astro or Gemlogis Belize segregator.

Does a diamond tester work on lab-grown diamonds?

A diamond tester reads a lab-grown diamond as a diamond, because it is one. To tell natural from lab-grown, use a segregator built for it, like the Gemlogis Belize or Belize PRO.

Which diamond tester is best for a jewelry store?

If you buy stones or take in trade, choose one that catches moissanite, like the Presidium Duo Tester PDT II. For quick in-house checks on your own stock, a thermal tester is enough.

Can a diamond tester identify colored gemstones?

Some can. The Presidium PGT II, Duo Tester PDT II and Gemlogis Astro help identify colored stones like ruby, sapphire and emerald. Basic diamond testers do not.

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