GEM STRUCTURE CHART
Compliments of
The International Gem Society
| Structure | Crystal axes and Angles | Optic Character | Refractive Indexes | Optic Sign | Pleochroism | Examples |
|---|---|---|---|---|---|---|
| Amorphous | No order No axes | Isotropic Singly refractive | 1 RI | None | None | Opal, Amber, Glass, Plastic |
| Isometric | 1 axis length All at 90° | Isotropic Singly refractive | 1 RI | None | None | Diamond, Spinel, Garnet |
| Tetragonal | 2 axis lengths All at 90° | Anisotropic Doubly refractive | 2 RI's | Uniaxial | May be dichroic | Zircon |
| Hexagonal | 2 axis lengths | Anisotropic Doubly refractive | 2 RI's | Uniaxial | May be dichroic | Quartz, Beryl, Apatite, Corundum, Tourmaline |
| Orthorhombic | 3 axis lengths All at 90° | Anisotropic Doubly refractive | 3 RI's | Biaxial | May be trichroic | Topaz, Zoisite, Peridot |
| Monoclinic | 3 axis lengths 2 axes at 90°; 1 oblique | Anisotropic Doubly refractive | 3 RI's | Biaxial | May be trichroic | Orthoclase, Spodumene |
| Triclinic | 3 axis lengths all axes oblique | Anisotropic Doubly refractive | 3 RI's | Biaxial | May be trichroic | Labradorite, Axenite |