Simulants: How to Structure the Problem

A simulant is defined by its imitative function, not by where it originated.

A diamond simulant is a material that imitates diamond in appearance or other observable properties but is not diamond material.

Two boundaries follow and must remain firm:

• a laboratory-grown diamond is not a simulant;
• a treated natural diamond is not a simulant.

Both are diamonds. They differ in origin or treatment history, not in material identity.

Identity first, origin second

For an unknown colorless stone, the professional sequence is not:

“Is it natural or laboratory-grown?”

The first question is:

“Is it even diamond?”

Only after diamond material identity has been confirmed does it make sense to ask whether the crystal is natural or laboratory-grown. Treatment, quality, provenance, and value follow after that.

This hierarchy prevents one of the most common logical errors in the modern market: automatically interpreting every unusual result as a laboratory-grown diamond, even though the sample may be moissanite, CZ, another simulant, or a complex material.

Nomenclature

ISO 18323:2015 governs permitted descriptors for diamonds, treated diamonds, synthetic diamonds, composites, and imitations. The standard was reconfirmed in 2026 and remains current.

The CIBJO Diamond Blue Book 2024 likewise classifies the following separately:

• diamond;
• treated diamond;
• synthetic/laboratory-grown diamond;
• imitation of diamond / diamond simulant;
• composite stone.

These documents are not identical to the law of every country, but they provide an important professional framework.

As of August 8, 2026, CIBJO’s proposal that only the term synthetic be preferred for industrially produced diamonds in a future revision had not yet been adopted in the Diamond Blue Book. It was to be considered as part of the Congress in September 2026. This book therefore distinguishes currently applicable nomenclature from a proposed change.

“Synthetic” must have a subject

The term synthetic does not automatically mean “synthetic diamond.”

Synthetic moissanite is laboratory-produced silicon carbide. Synthetic sapphire is corundum. Synthetic rutile is titanium dioxide.

Each of these materials may imitate diamond, but none becomes diamond as a result.

The material must therefore be identified by its full name.

An appearance clue is not identification

Simulants are successful precisely because they can look like diamond.

Possible visual clues include:

• very strong fire;
• facet doubling;
• different luster;
• worn facet junctions;
• characteristic inclusions;
• an unusual relationship between size and weight.

Such clues help select the next test. They do not replace confirmation of identity.

Physical properties as axes

Materials can be distinguished by a combination of properties:

• refractive index;
• single or double refraction;
• dispersion;
• density;
• hardness;
• thermal conductivity;
• electrical conductivity;
• spectral and Raman characteristics.

The key word is combination. One property is often insufficient.

The scratch test should be abandoned

Mohs hardness is important for understanding materials, but deliberately scratching an unknown stone or another object is a poor identification procedure.

The test may damage:

• the stone being examined;
• the setting;
• another material;
• a surface treatment.

Professional gemology favors nondestructive methods.

Thermal testers and the moissanite problem

Classic thermal diamond testers became important because diamond conducts heat very well, while many older simulants do not.

Synthetic moissanite changed that problem. Its thermal behavior can be sufficiently similar to diamond for some thermal-only devices to produce a diamond-like result.

It follows that:

“diamond” on a thermal tester is not proof of natural diamond origin and, depending on the device, is not necessarily final confirmation of diamond material identity either.

Modern combination testers may use additional electrical or other properties, but their validity always depends on the specific model, sample, and manner of use.

Screening and identification

Screening quickly separates samples compatible with a particular category from those requiring further analysis.

Identification confirms what the material is.

A result of refer or further testing required is therefore not a diagnosis. It means that the screening procedure did not resolve the item within its validated scope.

This is especially important for melee, mounted jewelry, and heterogeneous groups of stones.

Loose and mounted are not the same conditions

In a mounted stone, metal may interfere with:

• access to the girdle;
• determination of weight and density;
• observation of the pavilion;
• probe contact;
• optical paths.

The result of a method developed for loose stones should therefore not be applied uncritically to a mounted stone.

Composites and hybrids are not simple simulants

Some objects consist of several materials or layers. In that case, the question “what stone is this?” may be too simple.

It is necessary to describe:

• which components are present;
• their material nature;
• how they are joined or grown;
• which part produces the dominant optical effect.

This applies to assembled stones, surface-coated simulants, and natural/CVD hybrid diamonds.

A simulant is not fraud

Moissanite, CZ, and other simulants are legitimate products when described correctly.

Fraud occurs when:

• a simulant is sold as diamond;
• a laboratory document is falsified;
• an inscription is copied or counterfeited;
• material identity is deliberately concealed.

Professional terminology should therefore avoid the expression “fake diamond” as a general category. It is more precise to identify the material by its actual name.

Decision workflow

For an unknown diamond-like stone:

1. document the item and its condition;
2. distinguish the loose, mounted, and melee context;
3. perform a visual and microscopic examination;
4. collect nondestructive physical properties;
5. use a thermal or combination tester only within its validated scope;
6. exclude or confirm moissanite, CZ, and other simulants;
7. confirm material identity;
8. only if the material is diamond, proceed to natural/laboratory-grown origin;
9. then analyze treatment;
10. assess quality and value separately;
11. document uncertainty;
12. clearly communicate what is screening and what is a confirmed conclusion.

Chapter summary

• A diamond simulant looks like diamond but is not diamond material.
• A laboratory-grown diamond is not a simulant.
• A treated natural diamond is not a simulant.
• Material identity must precede natural/laboratory-grown origin.
• ISO 18323 and CIBJO provide a professional nomenclature framework but are not automatically the law of every jurisdiction.
• The term synthetic must be tied to the specific synthesized material.
• Appearance is useful for orientation but is not final identification.
• RI, refraction, dispersion, density, hardness, and conductivity operate as combined identification axes.
• The scratch test is not a professional method.
• A thermal-only tester may have difficulty with moissanite, and a tester does not determine natural/laboratory-grown origin.
• A screening result such as refer is not a diagnosis.
• A simulant is a legitimate product when the material is disclosed clearly and accurately.

[VISUAL 64.1: Hierarchy of questions—material identity → origin → treatment → quality/value]

[VISUAL 64.2: Natural diamond, laboratory-grown diamond, treated diamond, simulant, and composite—conceptual map]

[VISUAL 64.3: Screening versus identification—pass/refer/confirm logic]

[VISUAL 64.4: Decision tree for an unknown diamond-like stone]