Diamant vs. imitaties

Introduction

The word "simulant" has a precise meaning in gemology: a material that imitates the appearance of another gem but differs in composition, structure, and properties. A diamond simulant looks like a diamond in a jewellery case. It is not one.

This distinction matters because simulants are commercially widespread and, in some cases, intentionally marketed as diamond alternatives. Cubic zirconia dominates costume jewellery. Moissanite has carved out a position as a deliberate, disclosed alternative. White sapphire appears less often but still surfaces in budget engagement rings.

None of these materials is inherently deceptive — the problem arises only when a simulant is sold as, or mistaken for, a diamond. Understanding the differences protects your purchase and your confidence. This article compares the three most common simulants against diamond across the properties that matter: hardness, optical behaviour, density, and identifiability.

For the distinction between natural and laboratory-grown diamonds — which are chemically identical to each other and are not simulants — see Natural vs Lab-Grown — Definitions.

What Makes a Diamond a Diamond

Before comparing simulants, it helps to anchor what a diamond actually is. Diamond is crystalline carbon arranged in a face-centred cubic lattice — every carbon atom bonded to four neighbours in a rigid tetrahedral geometry. This structure gives diamond its defining properties:

• Hardness: 10 on the Mohs scale, the hardest known natural material.
• Refractive index: 2.417, responsible for the bright white light return gemologists call brilliance.
• Dispersion: 0.044, the splitting of white light into spectral colours — the source of fire.
• Thermal conductivity: Extremely high (~2,200 W/m·K), which is why diamond feels cold to the touch and why thermal testers work.
• Single refraction: Diamond is singly refractive (isotropic). Light passing through it does not split into two rays.

Any material claiming to be diamond must share all of these properties. Simulants share some — often impressively so — but never all. The gaps are where identification begins.

Cubic Zirconia (CZ)

Cubic zirconia is the crystalline cubic form of zirconium oxide (ZrO₂), first synthesised in the 1970s. It became the default diamond simulant because it is inexpensive to produce, optically lively, and available in large sizes and any colour.

Where CZ differs from diamond:

• Hardness: 8–8.5 Mohs. Hard enough for occasional wear, but noticeably softer than diamond. CZ scratches over time, and facet edges lose their sharpness — a well-worn CZ ring will look dull and abraded within a year or two of daily wear.
• Dispersion: 0.058–0.066, roughly 50 % higher than diamond. CZ throws more rainbow flashes than a diamond of similar cut. The effect is attractive in isolation but reads as excessive to anyone familiar with diamond's more balanced fire-to-brilliance ratio.
• Refractive index: 2.15–2.18, lower than diamond. CZ returns less white light (brilliance) and more dispersed colour — the visual signature is flashier but less crisp.
• Density: 5.6–6.0 g/cm³, roughly 70 % heavier than diamond (3.52 g/cm³). A CZ cut to diamond proportions in a given millimetre size will weigh substantially more. Conversely, a "one-carat" CZ is visibly smaller than a one-carat diamond because carat is a weight measure, not a size measure.
• Thermal conductivity: Low. A standard thermal diamond tester will immediately flag CZ as "not diamond." This is the fastest separation test.

Identification at the counter: Touch a thermal probe to the stone. CZ fails instantly. Under a loupe, look for rounded facet junctions (from softer polishing), orange-peel surface texture on older stones, and the absence of natural inclusions. CZ is typically flawless, which itself is a red flag — Flawless natural diamonds are vanishingly rare.

Moissanite

Natural moissanite — silicon carbide (SiC) — was first identified in 1893 by Henri Moissan in a meteor crater in Arizona. Natural crystals are microscopic and unsuitable for jewellery. All gem-quality moissanite is laboratory-created, primarily by Charles & Colvard and a growing number of manufacturers.

Moissanite is the simulant that most closely approaches diamond in durability, and the one most likely to fool a basic thermal tester.

Where moissanite differs from diamond:

• Hardness: 9.25 Mohs. Second only to diamond among gemstones. Moissanite resists scratching well and holds a polish over years of wear — a genuine advantage over CZ.
• Dispersion: 0.104, more than double diamond's 0.044. This is moissanite's most recognisable visual trait. Under direct light, moissanite produces strong, obvious rainbow flashes. In larger stones (above 1 carat equivalent), the effect is unmistakable to a trained eye.
• Refractive index: 2.648–2.691, actually higher than diamond. Moissanite is more brilliant than diamond in raw light return, but the quality of that light differs because of the extreme dispersion.
• Birefringence: Moissanite is doubly refractive — it splits each light ray into two. Under 10x magnification, looking through the crown at an angle, you can see doubled facet edges at the pavilion. Diamond, being singly refractive, never shows this doubling. This is the most reliable loupe test for separating moissanite from diamond.
• Thermal conductivity: High, close to diamond. A basic thermal-only tester may read moissanite as "diamond." Modern testers that also measure electrical conductivity correctly separate the two, because moissanite is an electrical semiconductor while diamond (with rare exceptions) is an insulator.
• Density: 3.21–3.22 g/cm³, slightly lower than diamond. A moissanite cut to diamond-equivalent millimetre dimensions will weigh about 10 % less.

Identification at the counter: Use a dual thermal–electrical tester (not a thermal-only model). Under a loupe, look for facet-edge doubling — rotate the stone and observe the pavilion facets through the table. If edges appear as parallel doubled lines, the stone is birefringent. Moissanite may also show faint needle-like inclusions parallel to the c-axis, though modern material is often inclusion-free.

White Sapphire

White sapphire is colourless corundum — aluminium oxide (Al₂O₃) — the same mineral family as ruby and blue sapphire, without the trace-element colouration. It is a natural gemstone, though most white sapphire used as a diamond alternative is heat-treated or lab-created.

Where white sapphire differs from diamond:

• Hardness: 9 Mohs. Excellent scratch resistance, though slightly below moissanite and well below diamond.
• Dispersion: 0.018, less than half of diamond's 0.044. White sapphire produces very little fire. Under light, it returns a clean but flat, almost waxy brightness without the spectral flashes that give diamond its liveliness.
• Refractive index: 1.762–1.770, significantly lower than diamond. White sapphire lacks both the brilliance and the fire of diamond. In side-by-side comparison, it looks glassy and subdued.
• Density: 3.99–4.00 g/cm³, about 13 % heavier than diamond.
• Thermal conductivity: Moderate. A thermal tester will correctly identify white sapphire as "not diamond."

Identification at the counter: White sapphire is the easiest common simulant to separate visually. It simply does not sparkle like a diamond. Under a loupe, look for curved growth lines (characteristic of Verneuil-process synthetics), gas bubbles in synthetic material, or fine silk (rutile needles) in natural stones. The thermal tester confirms the separation.

Properties Comparison

How to Tell Them Apart

For consumers, three practical tests cover every common simulant:

1. Thermal–Electrical Tester

A dual-mode tester (thermal + electrical) is the fastest instrument separation. Diamond reads as thermally conductive and electrically insulating. Moissanite reads as both thermally conductive and electrically conductive — the tester flags it. CZ and white sapphire fail the thermal test alone.

Thermal-only testers are unreliable for moissanite. If you are purchasing a tester, ensure it includes electrical conductivity measurement.

2. Loupe Examination (10x)

Under magnification, two observations separate simulants:

• Facet-edge doubling — visible in moissanite and (faintly) in white sapphire due to birefringence. Diamond and CZ, both singly refractive, never show doubled edges.
• Surface condition — CZ shows wear on facet edges and may display an orange-peel texture. Diamond facet edges remain crisp. White sapphire in Verneuil synthetics may show curved striae.

3. Weight and Proportion Check

If you have a millimetre gauge and a carat scale, weigh the stone and measure its diameter. A round brilliant diamond of 6.5 mm should weigh approximately 1.00 ct. A CZ of the same dimensions weighs roughly 1.70 ct. Moissanite of the same dimensions weighs about 0.88 ct. Significant deviation from expected diamond weight at a given size suggests a different material.

The Definitive Answer: A Grading Report

No simulant receives a GIA, HRD, or IGI diamond grading report. If a stone is accompanied by a verifiable report from a recognised laboratory, it is a diamond. If it is not, request one — or ask why it does not have one. For more on what a grading report contains, see What a Report Contains.

Frequently Asked Questions

Is moissanite a real diamond?

No. Moissanite is silicon carbide (SiC), a completely different material from diamond (carbon). It is the closest simulant in hardness (9.25 Mohs) and can fool basic thermal testers, but its strong birefringence and more than double diamond's dispersion give it away under informed examination.

How can I tell if my diamond is real or cubic zirconia?

The fastest test is a thermal probe — CZ fails instantly because its thermal conductivity is far lower than diamond's. Under a loupe, CZ shows rounded facet edges, may have orange-peel surface texture, and is typically flawless (which is itself a red flag, since flawless natural diamonds are extremely rare). CZ also weighs roughly 70% more than diamond at the same size.

Can a jeweller tell the difference between diamond and moissanite?

Yes, with the right equipment. A dual thermal-electrical tester separates them because moissanite is an electrical semiconductor while diamond is an insulator. Under 10x magnification, moissanite shows doubled facet edges due to birefringence — an effect diamond never displays.

Do simulants come with grading reports?

No simulant receives a GIA, HRD, or IGI diamond grading report. If a stone is accompanied by a verifiable report from a recognised grading laboratory, it is a diamond. Always request a report and verify it online before purchasing.

Summary

Diamond simulants — cubic zirconia, moissanite, and white sapphire — can approximate the look of a diamond in a display case, but each diverges in measurable, identifiable ways. CZ is the cheapest and easiest to detect, wearing poorly over time. Moissanite is the most durable and optically impressive alternative, though its excessive dispersion and birefringence give it away under informed examination. White sapphire lacks the optical life that defines diamond's appeal.

None of these materials is fraudulent in itself. The issue is always disclosure. A simulant sold as a simulant at a simulant's price is a legitimate product. A simulant presented as a diamond is a misrepresentation. The consumer's best protection is straightforward: insist on a grading report from a recognised laboratory and verify it online. For how to do this, see Online Report Verification.