The best diamond tester for your situation

Search "diamond tester" and you get three very different products under one name: pens that cost less than lunch, bench and pen multi testers in the low hundreds, and screening devices priced like a used car. They measure different things. Buying the wrong class is the most common mistake, and it usually happens because the cheap one looks almost identical in photos. How each type works is explained in how diamond testers work. This page is about which one to buy.

The three classes, side by side

Prices are retail listings we checked in October 2026 at US jewelry tool suppliers and big box marketplaces. Sale prices on multi testers move a lot, so treat the bands as where the market sits, not as quotes.

Class 1: thermal pens

These are the pens sold under names like "Diamond Selector II" on marketplaces, often between about $13 and $16 at Walmart in October 2026. They heat a small tip and light up a row of LEDs according to how fast the stone draws the heat away. Diamond draws it fast, glass and cubic zirconia draw it slowly.

The problem is moissanite. GIA's 1997 study of the material found that thermal probes react to it as if it were diamond. Moissanite is now sold everywhere as a diamond alternative, set in real gold, so a pen that reads it as diamond misses the imitation you are most likely to run into in a secondhand ring. It will still catch cubic zirconia and glass, which is why it feels like it works.

Who it is for

Honestly, almost nobody. If you are sorting a drawer of costume jewelry and only want to throw out the glass, it does that. For anything you would pay real money for, a thermal pen alone is not an answer. Our page on whether moissanite passes a diamond tester has the detail.

Class 2: multi testers

A multi tester runs the thermal check and an electrical check through the same probe. Moissanite conducts a small current; gem diamond does not. So the tester can say diamond, moissanite or neither in one touch. This is what most jewelers and pawn counters use for a quick answer.

Two models you will see everywhere

Specs in that table are the sellers' own descriptions, not our lab results. We have not benchmarked either unit and are not paid by either brand. Both are mainstream tools sold by jewelry suppliers such as SEP Tools and Otto Frei, which is the main reason to prefer them over an unnamed pen with a similar case.

What to check before you buy any multi tester

• Metal alert. If the tip touches a prong, the metal pulls heat away and a pen without an alert will happily call glass a diamond. On mounted stones this is the single most common false reading.
• A fine, retractable tip. Small stones and pavé need a tip narrower than the facet, or contact is unstable and readings jump around.
• A stated minimum stone size. If the seller cannot tell you the smallest stone it reads, assume it is not good on melee.
• A real supplier and warranty. Probe tips wear. A replaceable or warrantied tip matters more than an extra light on the panel.

Class 3: lab grown diamond screeners

No thermal or electrical tester can tell a lab grown diamond from a mined one, because both are diamond. Separating them takes instruments that read how the crystal responds to light. GIA's iD100 is the best known example: when it launched in 2017 it sold for $3,995 on an introductory offer, with a higher list price expected afterward. It passes natural colorless diamonds and refers everything else (simulants and lab grown stones) for further testing.

That is a business tool. If you are a buyer at a pawn shop or a jeweler taking in melee parcels, it pays for itself. If you have one ring, a lab report costs a fraction of the machine. Why a basic tester waves lab diamonds through is covered in do lab grown diamonds pass a diamond tester.

Which one you need

What to avoid

• "Professional" thermal pens with no moissanite mode. The word professional on a $15 listing means nothing. Check the product description for moissanite and metal detection, not the title.
• Testers sold as proof for resale. No handheld unit produces a result anyone else will rely on. Buyers will test the stone again themselves.
• Two-for-one bundles of the same thermal pen. Two pens that cannot see moissanite do not add up to one that can.
• Buying a tester instead of a lab report on a stone worth more than a few hundred dollars. The report answers what the tester cannot: origin, treatment and grade.

How much a reading is worth

Even the best multi tester answers one narrow question: what material is this? That rules out the cheap fakes, and that is all. Two diamonds that both beep can differ in value by a factor of ten because of color, clarity, cut and size. If the goal is money, the next step is a grade, and our diamond price calculator shows how much the 4Cs move a price once you know them.

Technique also matters as much as the device. Most wrong readings at home come from a dirty or warm stone, a tip touching metal or a weak battery. Our step by step guide to using a tester covers the routine that avoids them.

Our take

Skip the thermal pen. If you handle secondhand jewelry often enough to need your own tool, a mainstream multi tester with a metal alert is the minimum that answers the question you actually have. If you only have one ring, do not buy anything: walk it into a jeweler, or see where to get a diamond tested for the options and what each one costs.

Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.