- Visual observation: (10x lens) Look for color, luster, cut, doublet/triplet junctions, if any.
- Determine optic character: Single refractive (SR) / Double refractive (DR) / Anomalous Double refractive (ADR) / Aggregate (AGG).
- Spectrum: Many colorless stones may have diagnostic spectrum.
- Microscope: Inclusions may be diagnostic, but look for inclusions that differentiate natural and synthetic, doublet / triplet.
- Fluorescence: Look under shortwave and longwave for diagnostic colors.
- Immersion cell: Use immersion cell and high refractive index liquid to separate doublets/triplets.
- Refractometer: Confirm spectroscope reading with refractometer.
The colorless stones, which may resemble one another in appearance and values, are:
Diamond
- Hardness: 10
- Specific gravity: 3.52
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: negative refractive index, dispersion, luster, look at the girdle plane; cleavage, inclusions, weak to strong fluorescence in long wave (may be inert). Diamonds may be treated. Sapphire, synthetic sapphire, spinel, synthetic spinel, zircon, synthetic lithium niobate, synthetic rutile, synthetic yttrium aluminum garnet (YAG), synthetic gadolium gallium garnet (GGG), synthetic strontium titanate, synthetic cubic zircona (CZ), topaz, quartz, and glass may look like diamond. Standard / analytical techniques may be required to detect treatments / identify the stones.
Synthetic cubic zirconia
- Hardness: 8.5
- Specific gravity: 5.60 – 6.0
- Refractive index: 2.15 – 2.18 (average)
- Optic sign: SR
- Birefringence: -
- Other points: negative refractive index, dispersion, luster, orange flash on the pavilion, girdle, inclusions, fluorescence. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic strontium titanate
- Hardness: 5.5
- Specific gravity: 5.13
- Refractive index: 2.41
- Optic sign: SR
- Birefringence: -
- Other points: negative refractive index, dispersion, luster, girdle, inclusions, fluorescence, rounded facet edges. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic gadolium gallium garnet (GGG)
- Hardness: 6.5
- Specific gravity: 7.05
- Refractive index: 1.97
- Optic sign: SR
- Birefringence: -
- Other points: negative refractive index, dispersion, luster, girdle, color, inclusions, fluorescence (strong orange yellow (LW) and yellow (SW)), rounded facet edges. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic yttrium aluminum garnet (YAG)
- Hardness: 8.5
- Specific gravity: 4.58
- Refractive index: 1.83
- Optic sign: SR
- Birefringence: -
- Other points: negative refractive index, dispersion, luster, girdle, inclusions, fluorescence (variable). Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic rutile
- Hardness: 6.5
- Specific gravity: 4.25
- Refractive index: 2.61 – 2.90
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.287
- Other points: negative refractive index, strong dispersion, luster, doubling of back facets, inclusions, rounded facet edges. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic lithium niobate
- Hardness: 5.5
- Specific gravity: 4.64
- Refractive index: 2.21 – 2.30
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.09
- Other points: soft, negative refractive index, luster, inclusions. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Zircon (high type)
- Hardness: 7.5
- Specific gravity: 4.69
- Refractive index: 1.93 -1.99
- Optic sign: Uniaxial positive
- Birefringence: DR; 0.059
- Other points: dispersion, negative refractive index, luster, inclusions, doubling of back facets, spectrum. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Syntehic spinel
- Hardness: 8
- Specific gravity: 3.63
- Refractive index: 1.728
- Optic sign: SR/ADR
- Birefringence: -
- Other points: dispersion, vitreous, luster, inclusions, fluorescence. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic Moissanite
- Hardness: 9.25
- Specific gravity: 3.22
- Refractive index: 2.65-2.69
- Optic sign: DR
- Other points: Color, luster, fluorescence, inclusions, doubling of back facets. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Sapphire
- Hardness: 9
- Specific gravity: 4
- Refractive index: 1.76 – 1.77
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: vitreous, luster, inclusions, fluorescence. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Synthetic white (colorless) sapphire
- Hardness: 9
- Specific gravity: 4
- Refractive index: 1.76 – 1.77
- Optic sign: Uniaxial negative
- Birefringence: DR; 0.008
- Other points: vitreous, luster, inclusions, fluorescence. Gem quality stones are relatively clean. Standard / analytical techniques may be required to identify the stones.
Glass
- Hardness: 5.5
- Specific gravity: 3.70
- Refractive index: 1.60 – 1.66
- Optic sign: SR
- Birefringence: -
- Other points: Color, soft, inclusions (gas bubbles, swirls), luster, spectrum. Standard / analytical techniques may be required to identify the stones.
Assembled Stones
Doublets / Triplets
Diamond composites (diamond / diamond)
- Refractive index: 2.42
- Optic sign: SR/ADR
- Birefringence: -
- Other points: Immersion (Look for differences in color and luster between the sections), polishing marks.
Synthetic corundum or spinel on / synthetic strontium titanate
- Other points: Look for differences in color and luster between the sections, gas bubbles)
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