Prasiolite

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Prasiolite
Chemical composition SiO2 Silicon dioxide
Crystal system Trigonal
Habit Prismatic
Cleavage Poor
Fracture Conchoidal
Hardness 7
Optic nature Uniaxial +
Refractive index 1.544 - 1.553
Birefringence 0.009
Dispersion 0.013
Specific gravity 2.63 - 2.65
Lustre Vitreous
Pleochoism Weak to strong

Prasiolite is the green variety of quartz.
The color is caused by heat of either natural occuring amethyst (by exposure to sunlight or artificial) or by artificial heat treatment of irradiated amethyst. The former will give a more stable green color that is less prone to fading.

Synonyms: Prasiolita, greened amethyst, green quartz.
Misnomers: Green amethyst.

Prasiolite should not be confused with prasolite (a green variety of the chlorine group) and praseolite which is the green variety of cordierite.

Diagnostics

Chelsea Colour Filter

Reactions under incandescent light:

Irradiated green quartz: red.
Heat treated green quartz: green.