Olivine

(Mg,Fe)₂[SiO₄]

Orthosilicate

Crystal system: orthorhombic (+) (-)

Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7

Colour in plane polarised light: colourless
Relief: moderate 
Birefringence: 0.035; 2nd order pink

M100 Olivine 
North Carolina 
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage

L141 Gabbro 
L162 Basalt, porphyritic 
L172 Ultramafic cumulate

Olivine has two end-member compositions: forsterite, Mg₂[SiO₄], and fayalite, Fe₂[SiO₄]. The physical and optical properties detailed above refer to forsterite. Forsteritic olivine is a common constituent of mafic and ultra-mafic rocks.
Cracks are often filled with serpentinite alteration.

olivine
forsterite
fayallite