Gem Structure Chart

StructureCrystal axes and AnglesOptic CharacterRefractive IndexesOptic SignPleochroismExamples
AmorphousNo orderNo axesIsotropicSingly refractive1 RINoneNoneOpal, Amber, Glass, Plastic
Isometric1 axis lengthAll at 90°IsotropicSingly refractive1 RINoneNoneDiamond, Spinel, Garnet
Tetragonal2 axis lengths All at 90°AnisotropicDoubly refractive2 RI’sUniaxialMay be dichroicZircon
Hexagonal2 axis lengthsAnisotropicDoubly refractive2 RI’sUniaxialMay be dichroicQuartz, Beryl, Apatite, Corundum, Tourmaline
Orthorhombic3 axis lengthsAll at 90°AnisotropicDoubly refractive3 RI’sBiaxialMay be trichroicTopaz, Zoisite, Peridot
Monoclinic3 axis lengths 2 axes at 90°; 1 obliqueAnisotropicDoubly refractive3 RI’sBiaxialMay be trichroicOrthoclase, Spodumene
Triclinic3 axis lengths all axes obliqueAnisotropicDoubly refractive3 RI’sBiaxialMay be trichroicLabradorite, Axenite


Compliments ofThe International Gem Society

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