Gem Structure Table
How does a gem's crystal system affect its properties? Consult our gem structure table to learn more about selected gemstones.
By International Gem Society, updated on
Many gemstone properties stem from the crystal systems in which minerals form. The following gem structure table lists selected physical and optical characteristics based on mineral crystal systems, include characteristics of non-mineral amorphous gemstones.
| Crystal System | Crystal Axes and Angles | Isotropy/Anisotropy | Refractive Index | Optic Character | Pleochroism | Examples |
| Amorphous | No order, no axes | Isotropic | 1 RI | None | None | Opal, Amber, Glass, Plastic |
| Isometric | 1 axis length, all at 90° | Isotropic | 1 RI | None | None | Diamond, Spinel, Garnet |
| Tetragonal | 2 axis lengths, all at 90° | Anisotropic | 2 RIs | Uniaxial | May be dichroic | Zircon |
| Hexagonal | 2 axis lengths | Anisotropic | 2 RIs | Uniaxial | May be dichroic | Quartz, Beryl, Apatite, Corundum, Tourmaline |
| Orthorhombic | 3 axis lengths, all at 90° | Anisotropic | 3 RIs | Biaxial | May be trichroic | Topaz, Zoisite, Peridot |
| Monoclinic | 3 axis lengths, 2 axes at 90° (1 oblique) | Anisotropic | 3 RIs | Biaxial | May be trichroic | Orthoclase, Spodumene |
| Triclinic | 3 axis lengths, all axes oblique | Anisotropic | 3 RIs | Biaxial | May be trichroic | Labradorite, Axinite |
Gem structure table, compliments of The International Gem Society.
International Gem Society
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