Peridot

Peridot is the birthstone for August and is one of the most recognized minerals. While it is generally imagined to be green, there are also yellowish and colorless varieties.

Basically, peridot forms a solid solution of forsterite (Mg-dominant) and fayalite (iron). The former includes nearly colorless or gemstone-quality varieties, while the latter predominantly refers to brownish, lower-quality specimens. There are also "pallasitic peridots" which are not terrestrial, and Antarctic peridots, of which only small samples have been brought back in the past.

Name: Peridot
Japanese Name: Ganranshi (Olivine)
Chemical Composition: (Mg,Fe)₂SiO₄
Crystal System: Orthorhombic
Specific Gravity: 3.27–3.37
Mohs Hardness: 6.5
Cleavage: Indistinct to none (depending on direction)
Luster: Vitreous
Transparency: Transparent to translucent
Refractive Index: 1.654–1.703
Birefringence: 0.035–0.040
Color: Olive green, yellowish-green, brownish-yellow, colorless
Dispersion: 0.020