The 8 Best Diamond Testers

Anyone buying secondhand jewelry should start with the GemOro UltraTester 3+, because it checks for moissanite as well as diamond. Its tip glows a different color for diamond, moissanite, sapphire and metal, and GemOro says it catches the newer low-conductivity moissanite that slips past older testers. It runs on rechargeable batteries and comes with a case and stone holder. It can't tell lab-grown diamonds from mined ones, and the tip must be pushed fully in to read right.

Jewelers who check tiny side stones get the most from the Presidium Adamas. Its very fine tip reads colorless stones down to about a fiftieth of a carat, and an optional micro probe goes smaller still. A worn or bent tip can be swapped at the bench instead of sending the tester away. It runs on AAA batteries or USB. Presidium's own chart doesn't claim it catches low-conductivity moissanite, and it's a lot to spend for occasional use.

The Presidium Gem Tester II sits on a desk and shows its reading on an analog needle dial, the way jewelers' testers have worked for decades. Besides diamond and moissanite, the dial marks out about 16 kinds of gemstones, so it helps sort colored stones too. A slim probe on a cord does the touching after a guided heat calibration. That calibration can drift after a knock, and the boxy design keeps it on the bench.

Shop counters with a steady line of sellers suit the GemOro Testerossa. It speaks each result aloud in a choice of nine languages, so the person across the counter hears the verdict too. Its four-way color check covers diamond, moissanite, sapphire and metal, including newer low-conductivity moissanite. A charging dock and clip-on magnifier come in the case. It isn't built for spotting lab-grown diamonds, and the talking feature adds cost a home user may not need.

If modern moissanite is your main worry, look at SAM by Presidium. Presidium's comparison chart lists it as the one handheld in its range that catches the low-conductivity kinds, which older electrical testers can miss. It covers everything from tiny melee to large center stones and runs on a USB cord or AAA batteries. It has far less owner feedback behind it than Presidium's older testers, so its track record is short, and it won't flag a lab-grown diamond.

The Presidium Multi-Tester III puts a diamond check and a moissanite check into one chunky pen with a rubber grip. A colored light strip along its side shows the result, and it reads from either hand. The retractable tip keeps steady pressure on the stone and handles small colorless stones. It takes AAA batteries and comes in a zip case. It has to be calibrated before first use, and readings that wander when it's off are a common complaint.

For a jeweler who mostly needs to rule out cubic zirconia and glass, the Presidium Diamond Mate-C keeps things simple: one button, a row of lights and a fine heated tip. Its built-in battery recharges from the included wall adapter, so there are no cells to swap, and a metal alert sounds if the tip slips onto the setting. It only measures heat flow, so moissanite reads as diamond, and it needs a second tester to settle that question.

Thrift-store hunters and casual buyers can screen stones cheaply with the HMKIS Diamond Tester Pen. It flags cubic zirconia and glass, has a switch for small, medium and large stones, and comes with a lighted 60X loupe for looking at inclusions. Hold its metal panel with a bare finger while testing, and power it from a USB-C cord or a standard battery. The seller says plainly that it can't tell moissanite from diamond, and it won't read the tiniest stones.