| Structure | Crystal axes and Angles | Optic Character | Refractive Indexes | Optic Sign | Pleochroism | Examples |
|---|---|---|---|---|---|---|
| Amorphous | No orderNo axes | IsotropicSingly refractive | 1 RI | None | None | Opal, Amber, Glass, Plastic |
| Isometric | 1 axis lengthAll at 90° | IsotropicSingly refractive | 1 RI | None | None | Diamond, Spinel, Garnet |
| Tetragonal | 2 axis lengths All at 90° | AnisotropicDoubly refractive | 2 RI’s | Uniaxial | May be dichroic | Zircon |
| Hexagonal | 2 axis lengths | AnisotropicDoubly refractive | 2 RI’s | Uniaxial | May be dichroic | Quartz, Beryl, Apatite, Corundum, Tourmaline |
| Orthorhombic | 3 axis lengthsAll at 90° | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Topaz, Zoisite, Peridot |
| Monoclinic | 3 axis lengths 2 axes at 90°; 1 oblique | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Orthoclase, Spodumene |
| Triclinic | 3 axis lengths all axes oblique | AnisotropicDoubly refractive | 3 RI’s | Biaxial | May be trichroic | Labradorite, Axenite |
GEM STRUCTURE CHART
Compliments ofThe International Gem Society
Minerals that form in the tetragonal system form one of these three basic shapes. “Tetragonal System” by Donald Clark.
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