Moissanite, Cubic Zirconia, and Other Simulants

Chapter 64 established the methodology. This chapter applies it to the most important diamond simulants.

Today, the two most important groups for basic practical differentiation are:

• synthetic moissanite;
• cubic zirconia (CZ).

Both can be highly convincing colorless imitations, but their crystal structures and physical properties differ substantially from those of diamond—and from each other.

Synthetic moissanite: silicon carbide

Gem-quality moissanite is synthetic silicon carbide, SiC. It has been present in the jewelry market since the 1990s and has become one of the most successful diamond simulants.

Large gem-quality single crystals most commonly belong to the 4H and 6H SiC polytypes. This matters because “moissanite” should not be imagined as one single possible crystallographic variant without context.

Classic GIA data for gem-quality synthetic moissanite give approximately:

• refractive indices of about 2.648 and 2.691;
• dispersion of about 0.104;
• specific gravity of about 3.22;
• Mohs hardness of about 9.25.

These values provide a representative gemological framework, not a substitute for measuring the specific sample.

Why moissanite convincingly imitates diamond

Moissanite has:

• a high refractive index;
• very strong dispersion;
• high hardness;
• good durability;
• thermal conductivity that can confuse classic diamond testers.

Its fire can be stronger than diamond’s. This is visually attractive, but “more fire” is neither an identification test nor a value judgment.

Birefringence and facet doubling

Unlike cubic diamond, ordinary gem-quality 4H/6H moissanite is optically anisotropic.

When viewing geometry is suitable, doubled images of rear facet junctions may be seen through the stone. This doubling is one of the best-known classic signs of moissanite.

There are, however, two important qualifications:

1. doubling is not equally visible in every direction;
2. the absence of obvious doubling in one view is not proof that the stone is not moissanite.

Optical character must therefore be assessed systematically, not with one quick look through a loupe.

Thermal false positive

Early thermal probes were excellent at separating diamond from many simulants then in use. Moissanite revealed the limitation of that logic because its thermal conductivity can produce a diamond-like result on some thermal-only instruments.

Today’s testers are not all the same. Some combine thermal and electrical conductivity or use other logic.

It should therefore not be said that:

“moissanite always passes a diamond tester”

or that:

“a modern tester always distinguishes every sample without error.”

The claim must be tied to the specific instrument and its validated scope.

Coated moissanite

Surface technology further complicates identification. In 2024, GIA conducted a detailed investigation of colored coatings on synthetic moissanite, while earlier literature also documented diamond-related surface films on simulants.

This confirms the broader rule:

surface response may belong to the layer, while bulk material identity belongs to the substrate.

An unusual result therefore requires consideration of the sample’s architecture, not only one probe.

Cubic zirconia is not zircon

Cubic zirconia (CZ) is synthetic stabilized cubic zirconium oxide, approximately ZrO2 with appropriate stabilizers.

Zircon is the natural mineral ZrSiO4.

The similarity of the names is one of the most common terminological errors in gemology. The two materials are not synonymous.

Representative properties of CZ

Classic GIA literature for gem-quality CZ gives approximately:

• refractive index 2.15–2.18;
• dispersion 0.058–0.066;
• specific gravity 5.6–6.0;
• Mohs hardness 8.0–8.5;
• optical character: singly refractive in cubic material.

Composition and the proportion of stabilizers can shift individual properties, so the figures should be treated as ranges, not as an absolute identifying decimal signature.

Density is a strong practical difference

Diamond has a specific gravity of approximately 3.52. Moissanite is about 3.22, while CZ is significantly denser, often about 5.6–6.0.

CZ of the same geometric size will therefore typically have greater mass than diamond.

Conversely, if stones of the same mass are compared, CZ may have a smaller linear size.

This is a useful clue for loose stones with reliable measurements. Its application is far more limited in mounted jewelry.

Dispersion and brilliance are not the same

CZ has slightly greater dispersion than diamond, and moissanite has substantially greater dispersion still. Perceived fire, however, depends not only on a material constant but also on:

• cut;
• size;
• illumination;
• contrast;
• the observer’s position.

“Too much rainbow” may therefore be a clue, but it is not an identification conclusion.

Other simulants

Other materials have also been encountered historically and today as diamond imitations.

Colorless zircon may show strong brilliance and fire, but it has lower hardness and, depending on its structural state, pronounced birefringence.

Colorless sapphire and synthetic sapphire are substantially less refractive than diamond and have a different optical character.

Spinel, including synthetic colorless spinel, was historically used as an imitation.

Synthetic rutile is known for extreme dispersion and strong doubling.

YAG and GGG are important especially in the history of artificial diamond imitations.

Glass encompasses a broad group of compositions and qualities and should not be imagined as a single gemological material.

The full comparative property atlas belongs in Appendix F.

Three different logical profiles

For basic orientation, it is useful to think as follows:

Diamond

• very high RI;
• singly refractive;
• SG about 3.52;
• exceptionally high hardness;
• very high thermal conductivity.

Moissanite

• even higher refractive-index values;
• doubly refractive in typical 4H/6H gem materials;
• SG about 3.22;
• stronger dispersion;
• very high hardness;
• thermal response may resemble diamond.

CZ

• lower RI than diamond and moissanite;
• singly refractive;
• substantially greater density;
• slightly greater dispersion than diamond;
• lower hardness;
• thermal response typically does not mimic diamond in the way moissanite can.

This is not a substitute for laboratory identification. It is a map for reasoning.

A false inscription does not change the material

GIA has documented several synthetic moissanite samples with counterfeit GIA inscriptions corresponding to existing reports for natural diamonds.

Such a case demonstrates why the following rule applies:

report number + inscription ≠ confirmed physical match to the stone.

Dimensions, material properties, optical character, and, when needed, spectroscopy must be mutually consistent.

Counterfeits are examined in detail in Chapter 74.

Mounted and melee limitations

In a mounted stone, the setting may conceal:

• the rear facets needed to observe doubling;
• the girdle;
• access for certain probes;
• the geometry required to estimate weight and density.

With melee, high throughput becomes the challenge. A method that is excellent for one loose stone may be impractical for thousands of small stones.

Validated screening systems and clearly defined refer logic are therefore used for large batches, as discussed in detail in Chapter 73.

Final workflow

When diamond, moissanite, or CZ is suspected:

1. examine the item and the condition of its surface;
2. establish the loose, mounted, or melee context;
3. observe luster, facet junctions, fire, and possible doubling;
4. use an appropriate thermal or combination tester only for screening;
5. for a loose stone, use the available physical measurements;
6. determine whether the body of properties is compatible with moissanite or CZ;
7. do not rely on an inscription or document alone;
8. if the results remain contradictory, proceed to Raman, FTIR, or another appropriate laboratory procedure;
9. only after diamond identity is confirmed, analyze natural/laboratory-grown origin;
10. keep treatment and commercial quality as separate questions.

Chapter summary

• Synthetic moissanite is silicon carbide, not a laboratory-grown diamond.
• Gem-quality moissanite is most commonly associated with 4H and 6H SiC polytypes.
• Representative moissanite has an RI of about 2.65–2.69, SG of about 3.22, dispersion of about 0.104, and Mohs hardness of about 9.25.
• Moissanite is typically doubly refractive and may show facet doubling.
• Doubling is not equally visible from every direction and is insufficient as a standalone test.
• A thermal-only diamond tester may produce a diamond-like result on moissanite.
• Cubic zirconia and natural zircon are completely different materials.
• CZ typically has an RI of about 2.15–2.18, SG of about 5.6–6.0, and Mohs hardness of about 8.0–8.5.
• CZ is cubic and optically singly refractive.
• The high density of CZ and birefringence of moissanite are useful complementary clues.
• Zircon, sapphire, spinel, synthetic rutile, YAG, GGG, and glass may also serve as diamond imitations.
• Final identification follows from a consistent body of properties, not from one visual characteristic, tester, or inscription.

[VISUAL 65.1: Diamond, moissanite, and CZ—comparative map of RI, refraction, SG, dispersion, and hardness]

[VISUAL 65.2: Facet doubling in moissanite—the optical principle without turning it into a universal standalone test]

[VISUAL 65.3: Equal geometry, different mass—diamond versus CZ]

[VISUAL 65.4: Diamond-like stone workflow—diamond / moissanite / CZ / other → material confirmation]