Quick answer: Choose a dual thermal and electrical conductivity tester from a trusted brand such as Presidium or Gemlogis: it separates diamond from moissanite in one pass. Add a synthetic diamond screener if you handle goods where lab-grown stones are a risk. Thermal-only pens are cheapest but cannot catch moissanite.
Choosing the right diamond tester depends on what you need to detect: a natural diamond, a moissanite, or a lab-grown (CVD/HPHT) diamond. No single handheld device does all three perfectly, so the best choice comes down to matching the technology to your workflow. This guide from the Sachi gemology team explains how testers work and which instrument fits your needs.
What does a diamond tester actually measure?
Most testers use one of two principles. Thermal conductivity testers measure how fast heat moves through a stone — diamond conducts heat exceptionally well, so it separates diamond from most simulants. Electrical conductivity testers add a second probe to distinguish moissanite, which can fool a thermal-only tester. Understanding this difference is the single most important factor in your purchase.
Do I need a moissanite tester too?
Yes, if you handle modern jewelry. Moissanite has thermal properties close to diamond and will read as "diamond" on a basic thermal probe. A combined thermal + electrical instrument such as the Presidium Multi Tester III separates diamond, moissanite, and simulants in one device. For diamond-only checking on a budget, the Presidium Diamond Mate-A is a reliable thermal tester.
What about lab-grown and CVD diamonds?
This is the critical limitation buyers miss: standard testers cannot tell a lab-grown diamond from a natural one — both are real diamond and behave identically on thermal/electrical probes. To flag lab-grown material you need a dedicated CVD tester or a synthetic screener:
- GemTrue APEX — handheld synthetic diamond tester
- Gemlogis Belize Pro CVD Tester — handheld CVD detection
- DRC Guardian — synthetic diamond screener for loose stones
- DRC Sentinel — screener for larger batches
Tester vs. screener — what's the difference?
A tester confirms whether a stone is diamond, moissanite, or simulant. A screener sorts natural from lab-grown/synthetic diamond, typically flagging "pass" (likely natural) or "refer" (needs lab confirmation). Most professionals use both in a layered workflow. For the full breakdown, see our companion guide, How to Tell if a Diamond Is Real, Lab-Grown, or Moissanite.
Recommended picks by use case
- Retail counter / quick checks: Presidium Diamond Mate-A
- Diamond + moissanite separation: Presidium Multi Tester III
- Lab-grown detection (handheld): GemTrue APEX or Gemlogis Belize Pro CVD
- Batch screening: DRC Guardian / DRC Sentinel
Browse the full range in our Diamond & Gem Testers and Synthetic Diamond Screeners collections.
Frequently asked questions
How accurate are handheld diamond testers?
Thermal/electrical testers are highly reliable for separating diamond, moissanite, and common simulants when used correctly on a clean, dry stone at room temperature. They cannot, however, distinguish natural from lab-grown diamond.
Do testers need calibration?
Quality testers self-calibrate on power-up. Always let the unit warm up, test on a clean stone, and avoid readings immediately after handling, since body heat can skew thermal results.
Can a tester be fooled by coated or treated stones?
Surface coatings and some treatments can affect readings. For high-value stones, pair a tester with magnification and, when in doubt, refer to a certified lab.
What currency and shipping do you offer?
All prices are listed in Thai Baht (THB). We ship worldwide, including the USA, Europe, and across Thailand.
Which single device covers the most cases?
The Presidium Multi Tester III covers diamond, moissanite, and simulants; add a screener like the DRC Guardian for lab-grown detection.
Written by the Sachi Gems & Tools team — gemological instrument specialists based in Bangkok.