Optical Properties¶
Refractive index, birefringence, pleochroism, dispersion, and other optical properties.
Interactive Version
For detailed data tables and interactive content, visit gemmology.dev/learn/optical-properties.
Refractive Index (RI)¶
The refractive index measures how much light slows down when entering a gemstone.
| Gemstone | RI Range | Optic Character |
|---|---|---|
| Diamond | 2.417 | Isotropic |
| Corundum | 1.762-1.770 | Uniaxial - |
| Spinel | 1.712-1.736 | Isotropic |
| Quartz | 1.544-1.553 | Uniaxial + |
Birefringence¶
Birefringence is the difference between maximum and minimum RI values in anisotropic crystals.
Formula: Birefringence = RI(max) - RI(min)
| Birefringence | Gemstones |
|---|---|
| None (0.000) | Diamond, Spinel, Garnet |
| Low (0.004-0.010) | Quartz (0.009), Beryl (0.006) |
| High (>0.020) | Peridot (0.036), Zircon (0.059) |
Doubling Effect
High birefringence causes visible "doubling" of back facet edges. This is diagnostic for zircon, sphene, and peridot.
Pleochroism¶
Pleochroism is the absorption of different colours in different crystal directions.
| Gemstone | Strength | Colours |
|---|---|---|
| Ruby | Strong | Purple-red / Orange-red |
| Tourmaline | Strong | Variable |
| Tanzanite | Strong | Blue / Purple / Bronze |
Dispersion¶
Dispersion is the separation of white light into spectral colours ("fire").
| Gemstone | Dispersion | Fire |
|---|---|---|
| Diamond | 0.044 | Strong |
| Sphene | 0.051 | Very strong |
| Demantoid | 0.057 | Very strong |
Data source: learn/optical-properties.yaml