Optical Properties
Refractive index, birefringence, pleochroism, dispersion, and other optical properties of gemstones.
Introduction
Optical properties are the most important diagnostic features for gem identification.
The refractometer, polariscope, and dichroscope are essential tools for measuring
these properties.
Refractive Index (RI)
The refractive index measures how much light slows down when entering a gemstone.
It's calculated as the ratio of light speed in vacuum to speed in the material.
A standard gemmological refractometer can measure RI values from approximately
1.35 to 1.81 (limited by the contact liquid). Gems with higher RI require
alternative methods.
| Gemstone | RI Range | Optic Character |
|---|---|---|
| Diamond | 2.417 | Isotropic |
| Corundum | 1.762-1.770 | Uniaxial - |
| Spinel | 1.712-1.736 | Isotropic |
| Tourmaline | 1.624-1.644 | Uniaxial - |
| Topaz | 1.609-1.617 | Biaxial + |
| Quartz | 1.544-1.553 | Uniaxial + |
| Beryl | 1.570-1.590 | Uniaxial - |
Birefringence
Birefringence is the difference between maximum and minimum RI values in
anisotropic (non-cubic) crystals. Light entering the stone splits into two
rays traveling at different speeds.
Formula: Birefringence = RI(max) - RI(min)
Example (Quartz): 1.553 - 1.544 = 0.009
| Birefringence | Gemstones |
|---|---|
| None (0.000) | Diamond, Spinel, Garnet (isotropic) |
| Low (0.004-0.010) | Quartz (0.009), Beryl (0.006) |
| Medium (0.010-0.020) | Topaz (0.010), Tourmaline (0.018) |
| High (0.020-0.050) | Peridot (0.036), Zircon (0.059) |
| Very High (>0.050) | Calcite (0.172), Sphene (0.134) |
Doubling Effect
Optic Character and Sign
The polariscope and conoscope reveal a gem's optic character.
| Character | Crystal Systems | RI Values |
|---|---|---|
| Isotropic | Cubic | Single RI (n) |
| Uniaxial | Hexagonal, Trigonal, Tetragonal | Two values (ω, ε) |
| Biaxial | Orthorhombic, Monoclinic, Triclinic | Three values (α, β, γ) |
Pleochroism
Pleochroism is the absorption of different colours in different crystal directions.
Viewed with a dichroscope, anisotropic coloured gems show different colours.
| Gemstone | Pleochroism | Colours |
|---|---|---|
| Ruby | Strong | Purple-red / Orange-red |
| Sapphire (blue) | Strong | Violet-blue / Green-blue |
| Tourmaline | Strong | Variable (often dark/light) |
| Tanzanite | Strong (trichroic) | Blue / Purple / Bronze |
| Peridot | Weak | Yellow-green / Green |
| Aquamarine | Weak | Near colourless / Blue |
Dispersion
Dispersion is the separation of white light into spectral colours ("fire").
It's measured as the difference in RI between red (B) and violet (G) wavelengths.
| Gemstone | Dispersion (B-G) | Fire Quality |
|---|---|---|
| Diamond | 0.044 | Strong |
| Sphene | 0.051 | Very strong |
| Demantoid garnet | 0.057 | Very strong |
| Zircon | 0.039 | Strong |
| Spinel | 0.020 | Moderate |
| Quartz | 0.013 | Low |
Absorption Spectrum
The absorption spectrum shows which wavelengths of light are absorbed by a gem.
Viewed through a spectroscope, absorption bands appear as dark lines or bands.
Diagnostic Spectra: