Crystal Obsidian

Category	Volcanic glass
Formula (repeating unit)	70–75% SiO2, plus MgO, Fe3O4
Identification
Color	Deep black or blackish green
Fracture	Conchoidal
Mohs scalehardness	5–6[1]
Luster	Vitreous
Specific gravity	~2.4[2]
Optical properties	Translucent
References	[3]