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

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.