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

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Minerals that form in the tetragonal system form one of these three basic shapes. “Tetragonal System” by Donald Clark.