When a jeweler holds a pen-like device to a diamond and it beeps, what's actually happening? Diamond testing is both simpler and more nuanced than most people realize. Here's how the tools work — and their surprising limitations.
The Standard Diamond Tester: Thermal Conductivity
The most common diamond tester measures thermal conductivity. Diamonds conduct heat exceptionally well — better than almost any gemstone. When the tester's probe touches a stone, it measures how quickly heat dissipates. A genuine diamond (or moissanite, which also conducts heat well) reads as "diamond." Most imitations — glass, cubic zirconia, quartz — conduct heat poorly and read as "not diamond."
Electrical Conductivity Testers
A second type of tester checks electrical conductivity, which is different among gemstones. This device is especially useful for distinguishing moissanite from diamond — moissanite conducts electricity in a way diamonds don't, so it helps separate the two. A combined thermal-plus-electrical tester is the most accurate consumer-available check.
The Limitation: Moissanite Fools Basic Testers
Here's the catch most people miss: moissanite conducts heat well enough to fool a basic thermal diamond tester. A standard tester will often read moissanite as "diamond." That's why jewelers use electrical testers too. If you only use a basic heat tester, a high-quality moissanite may slip through — which is why professional verification goes beyond the pen.
Magnification: The Loupe and Microscope
Beyond conductivity, jewelers examine stones under a 10x loupe or a microscope. Genuine diamonds (mined or lab-grown) show characteristic internal features — inclusions, or the growth patterns of lab-created stones. Glass, cubic zirconia, and most simulants look "too clean and perfect." Expert magnification can also spot treatments or evidence of a mismatch with a certificate.
The Definitive Check: Certification
The most reliable way to verify a diamond isn't a handheld tester at all — it's an independent grading report from GIA or IGI. A certified diamond has been analyzed by a lab using specialized equipment and given a report confirming carat, color, clarity, cut, and origin (mined vs lab-grown). For anything of value, the certificate is the gold standard.
Why It Matters
Whether you're buying, insuring, or selling, knowing exactly what your stone is protects you. A quick tester is a good first line of defense, but professional tools plus an independent certificate give certainty. At Lara Jewels, every diamond we sell is certified — so you never have to wonder what's on your finger.
