Gem Identification
What this domain tests
Gem Identification tests instrument use and the optical, physical, and chemical clues that separate species, natural from synthetic, and treated from untreated stones.
Core rules
- Refractometer: gives RI and, for anisotropic stones, birefringence (highest minus lowest RI). A single reading suggests isotropic material (cubic or amorphous) or a reading obscured by cut; confirm with the polariscope.
- Polariscope: distinguishes isotropic from anisotropic, shows strain and pleochroism; with a conoscope, optic character (uniaxial or biaxial) and sign.
- Dichroscope: shows pleochroic colors in anisotropic stones.
- Spectroscope: absorption bands identify chromophores, such as chromium lines near 694 and 693 nm in ruby and emerald, and iron-related bands in blue sapphire.
- UV lamp: fluorescence and phosphorescence; the Chelsea filter is a supplementary test.
- Microscope: inclusions, growth zoning, curved striae, flux fingerprints, and two-phase inclusions are key natural-versus-synthetic evidence.
- SG by hydrostatic weighing: SG = weight in air ÷ (weight in air − weight in water). Heavy liquids are hazardous and less used.
- Crystal systems: cubic (isotropic), tetragonal and hexagonal (uniaxial), orthorhombic, monoclinic, and triclinic (biaxial).
Worked example
A colorless stone gives a single RI reading of 2.15, is isotropic under the polariscope, and has SG 5.6. Diamond would be RI 2.417 and SG 3.52; glass is near RI 1.50–1.70 and SG 2.5; cubic zirconia matches RI about 2.15 and SG about 5.6. The consistent isotropic single refraction, RI, and SG identify cubic zirconia, not diamond. A thermal tester would support the separation but is not the primary evidence.
Traps
Do not rely on one instrument. Do not confuse birefringence with dispersion. Do not call a singly refractive stone diamond without RI and SG. Do not overlook synthetic growth features or treatment evidence. Do not use destructive tests when a non-destructive one answers the question.