How does a gem's crystal system affect its properties? Consult our gem structure table to learn more about selected 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.
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| 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.
