Properties of Diamond Simulants
Gem-grade synthetic diamonds rarely appear in jewelry. However, diamond simulants often do, as side stones as well as centerpieces. Identifying these look-alike materials, whether glass, cubic zirconia, or natural gems that resemble diamonds, is an important skill for gemologists.
The chart below compares the physical and optical properties of natural diamonds with those of frequently encountered diamond simulants. Hopefully, this information can help you distinguish diamonds from their imitators.
Properties of Natural Diamonds vs. Diamond Simulants
| Properties | Natural Diamond Type Ia | Natural Diamond Type Ib | Natural Diamond Type IIa | Natural Diamond Type IIb | Moissanite | Cubic Zirconia |
|---|---|---|---|---|---|---|
| RI | 2.417 | 2.417 | 2.417 | 2.417 | 2.648 – 2.691 | 2.15 – 2.18 |
| Birefringence | None | None | None | None | .043 | None |
| Dispersion | .044 | .044 | .044 | .044 | .009 – .104 | .058 – .066 |
| Optic Character | Isotropic | Isotropic | Isotropic | Isotropic | Uniaxial + | Isotropic |
| Pleochroism | None | None | None | None | None | None |
| Read Through | None | None | None | None | None | Slight |
| Luster | Adamantine | Adamantine | Adamantine | Adamantine | Adamantine | Subadamantine |
| SG | 3.515 | 3.515 | 3.515 | 3.515 | 3.17 – 3.22 | 5.56 – 6.00 |
| Hardness | 10 | 10 | 10 | 10 | 9 ¼ | 8 – 8 ½ |
| Cleavage | Perfect, four directions | Perfect, four directions | Perfect, four directions | Perfect, four directions | None | None |
| Absorption Spectra | Dark line at 415 and 478 nm, often more | Increasing absorption towards blue end | Variable, usually 504 | Variable | No sharp bands, strong absorption below 425 | Not diagnostic |
| UV | Colorless stones, inert to strong, usually blue (LW) Weaker (SW) | Variable, can be inert, usually blue or yellow | Variable, can be inert, often red | Inert but phosphoresce after exposure to SW UV | Brownish yellow, (LW) | Greenish/yellow or orangish/yellow (LW) Yellow (SW) Colored CZ varies |
| Girdle Surface | Waxy, polished or granular | Waxy, polished or granular | Waxy, polished or granular | Waxy, polished or granular | Frosted, polished or striated | Frosted or polished |
| Facet Junctions | Sharp | Sharp | Sharp | Sharp | Sharp or slightly rounded | Sharp or slightly rounded |
| Electrical Test | None | None | Very Weak | Strong | Frequent, variable intensity | None |
| Thermal Test | “Diamond” | “Diamond” | “Diamond” | “Diamond” | “Diamond” | “Simulant” |
| Inclusions | Bearding, crystals, planar graining | Bearding, crystals, planar graining | Bearding, crystals, planar graining | Bearding, crystals, planar graining | Bearding, crystals, planar graining | Gas bubbles, negative crystals, unmelted powder |
| Properties | GGG | YAG | Synthetic Spinel* | White Sapphire | White Topaz | Glass |
|---|---|---|---|---|---|---|
| RI | 1.970 – 2.03 | 1.823 – 1.843 | 1.720 – 1.740 | 1.757 – 1.779 | 1.609 – 1.637 | 1.40 – 1.70 |
| Birefringence | None | None | None | .008 – .010 | .008 – .010 | None |
| Dispersion | .038 – .045 | .028 | .020 | .018 | .014 | .009 – .098 |
| Optic Character | Isotropic | Isotropic | Isotropic | Uniaxial – | Biaxial + | Amorphous |
| Pleochroism | None | None | None | None if totally colorless | None if totally colorless | None |
| ReadThrough | Moderate | Strong | Strong | Strong | Strong | Strong |
| Luster | Vitreous to subadamantine | Vitreous to subadamantine | Vitreous to subadamantine | Vitreous to subadamantine | Vitreous | Vitreous |
| SG | 7.01 – 7.15 | 4.50 – 4.60 | 3.52 – 3.67 | 3.90 – 4.10 | 3.49 – 3.57 | 2.30 – 4.50 |
| Hardness | 6.5 – 7 | 8.25 – 8.5 | 7.5 – 8 | 9 | 8 | 5 – 6 |
| Cleavage | None | None | None | None | Perfect, 1 direction | None |
| Absorption Spectra | Not diagnostic | Not diagnostic | Not diagnostic | Not diagnostic | Not diagnostic | Not diagnostic |
| UV | Moderate to strong, pinkish orange (SW) | Inert to moderate orange, (LW) Inert to weak orange, (SW) | Occasionally, weak green (LW) Moderate to strong chalky or greenish blue, (SW) | Natural: Inert to moderate red to orange (LW & SW)Synthetic: inert to weak, blueish white (SW) | Not diagnostic | Not diagnostic |
| Girdle Surface | Frosted or polished | Frosted or polished | Frosted or polished | Frosted or polished | Frosted or polished | Frosted or polished |
| Facet Junctions | Slightly rounded | Sharp or slightly rounded | Sharp or slightly rounded | Sharp or slightly rounded | Sharp or slightly rounded | Rounded |
| Electrical Test | None | None | None | None | None | None |
| Thermal Test | “Simulant” | “Simulant” | “Simulant” | “Simulant” | “Simulant” | “Simulant” |
| Inclusions | Gas bubbles | Gas bubbles, curved striae | Gas bubbles | Natural: Silk, crystals, fingerprint, hexagonal banding. Synthetic: bubbles, curved striae, flux | 2 & 3 phase | Bubbles, swirl lines, concave facets |
Compliments of the International Gem Society.
Notes
* Colorless natural spinels are extremely rare. Should you encounter a colorless spinel as a diamond simulant, it’s very likely synthetic.
For information on diamond color treatments, see our article on HPHT diamonds.
