Diamond vs Moissanite: How Gemologists Tell the Difference (And the Tools They Use)
Introduction
To the naked eye, diamond and moissanite are virtually indistinguishable. Both are colourless, both sparkle brilliantly, and both are used extensively in fine jewellery. Yet they are fundamentally different stones — one a naturally occurring carbon crystal, the other a lab-created silicon carbide — with very different values.
For jewellers, gemologists, pawnbrokers, and estate buyers, being able to tell them apart isn't just useful — it's essential. In this guide, we explain the key differences between diamond and moissanite, the limitations of traditional testing methods, and the professional tools that give you a definitive answer.
Diamond vs Moissanite: Key Differences
| Property | Diamond | Moissanite |
|---|---|---|
| Composition | Carbon (C) | Silicon carbide (SiC) |
| Origin | Natural or lab-grown | Almost exclusively lab-created |
| Hardness (Mohs) | 10 | 9.25 |
| Refractive index | 2.417 | 2.65–2.69 |
| Thermal conductivity | Very high | High (similar to diamond) |
| Electrical conductivity | Non-conductive (usually) | Conductive |
| Birefringence | None | Strong (double refraction) |
| Value | High | Significantly lower |
The challenge for testers is that moissanite was specifically engineered to mimic diamond's optical and thermal properties — which is why older testing methods often fail to distinguish between them.
Why Traditional Diamond Testers Fail with Moissanite
The most widely used diamond testers work by measuring thermal conductivity. Diamond is an exceptional conductor of heat — far better than most gemstones — so a thermal tester can quickly distinguish diamond from cubic zirconia (CZ), glass, or synthetic sapphire.
The problem? Moissanite has thermal conductivity close to diamond. Many thermal-only testers will read moissanite as diamond, giving a false positive. This was a significant issue when moissanite first entered the market in the late 1990s, and it remains a pitfall for anyone relying on older or entry-level equipment.
If you're using a basic thermal diamond tester and it reads positive, that does not confirm you have a diamond. It confirms you have a stone with high thermal conductivity — which includes moissanite.
How to Tell Diamond from Moissanite: The Right Methods
1. Electrical Conductivity Testing
This is the most reliable method for distinguishing diamond from moissanite. Diamond does not conduct electricity (with the exception of some rare Type IIb diamonds). Moissanite does.
A combined thermal and electrical tester — sometimes called a diamond/moissanite tester — tests both properties simultaneously. If the stone conducts electricity, it's moissanite (or another conductive material). If it doesn't, and it has high thermal conductivity, it's almost certainly diamond.
This dual-test approach is now the professional standard for distinguishing the two stones.
2. Loupe Examination (Birefringence)
Moissanite has strong double refraction (birefringence), meaning light passing through it splits into two rays. Under a 10x loupe, you can often see this as a doubling of the back facets when viewed through the table of the stone.
Diamond is singly refractive — you won't see this doubling effect. This is a useful secondary check, though it requires practice and a good quality loupe to observe reliably.
3. Spectroscopic Analysis
For the highest level of certainty — particularly for high-value stones — spectroscopic analysis (Raman spectroscopy or FTIR) provides a definitive identification. This is laboratory-grade equipment used by gemmological institutes such as the GIA and Gemmological Association of Great Britain (Gem-A).
For most jewellers and pawnbrokers, a quality dual-tester combined with loupe examination is sufficient for day-to-day identification.
4. UV Fluorescence
Many diamonds fluoresce blue under long-wave UV light. Moissanite typically shows a different fluorescence pattern — often yellowish-green or none at all. This is a useful supplementary check but not definitive on its own, as some diamonds show no fluorescence and fluorescence varies widely.
Recommended Tools for Diamond and Moissanite Testing
Combined Diamond/Moissanite Testers
A quality combined tester is the essential tool for any jeweller or pawnbroker handling diamonds. Look for a device that tests both thermal conductivity and electrical conductivity simultaneously, with a clear readout distinguishing between diamond, moissanite, and other stones.
These testers are compact, battery-operated, and straightforward to use — making them ideal for counter-top use in a retail or pawnbroking environment.
Gemological Loupes
A 10x triplet loupe is the standard tool for visual gemstone examination. For moissanite identification specifically, look for a loupe with good colour correction (achromatic or apochromatic) to clearly observe facet doubling. A 10x loupe is also essential for examining inclusions, assessing cut quality, and reading laser inscriptions on certified stones.
UV Lamps
A long-wave UV lamp (365nm) is a useful addition to any gemological toolkit. While not definitive for diamond/moissanite identification, UV fluorescence provides a quick supplementary data point and is invaluable for identifying treatments in coloured stones and detecting repairs in pearls.
Practical Tips for Jewellers and Pawnbrokers
- Never rely on a thermal-only tester for diamond verification. Always use a combined thermal/electrical tester.
- Test before purchasing. If you're buying a stone or piece of jewellery, test it before agreeing a price — not after.
- Check the setting. Moissanite is often set in white gold or platinum to enhance its diamond-like appearance. The metal type alone tells you nothing about the stone.
- Look for certificates. GIA, IGI, and HRD certificates are reliable for natural and lab-grown diamonds. Be cautious of certificates from lesser-known labs.
- Use multiple methods. No single test is infallible. A combination of electrical testing, loupe examination, and UV fluorescence gives you the highest confidence.
What About Lab-Grown Diamonds?
It's worth noting that lab-grown diamonds are chemically and physically identical to natural diamonds. A diamond/moissanite tester will read a lab-grown diamond as diamond — correctly. The distinction between natural and lab-grown diamonds requires specialist equipment (such as a DiamondView or HPHT detection device) and is a separate identification challenge.
If you're handling high-value stones and need to distinguish natural from lab-grown, we recommend sending the stone to a gemmological laboratory for certification.
Frequently Asked Questions
Can a jeweller always tell diamond from moissanite?
Not with the naked eye, and not with a thermal-only tester. A combined thermal/electrical tester is required for reliable identification.
Is moissanite a fake diamond?
Moissanite is a genuine gemstone in its own right — it's not a fake, but it is frequently sold as a diamond alternative. The issue arises when it's misrepresented as diamond, which is why testing matters.
How much does a diamond/moissanite tester cost?
Quality combined testers are available from around £30–£150 depending on the model and features. For professional use, investing in a mid-range or higher-end model is worthwhile for reliability and longevity.
Can I test a stone while it's set in jewellery?
Yes, in most cases. Ensure the probe makes clean contact with the stone's surface and that you're not touching the metal setting, which can affect electrical conductivity readings.
Shop Gemological Testing Tools at Dynagem
Dynagem stocks a range of professional gemstone testing equipment, including combined diamond/moissanite testers, gemological loupes, and UV lamps. All products are sourced from trusted manufacturers and are used by jewellers, pawnbrokers, and gemologists across the UK.
Browse our gemological tools collection or contact us for expert advice on the right tools for your needs.
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