Snowflake Obsidian is a black variety of obsidian with white or grey "snowflakes" which are composed of radiating needles of Cristobalite, a type of quartz.
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) causes a high viscosity, which, upon rapid cooling, results in a natural glass forming from the lava. The inhibition of atomic diffusion through this highly viscous lava explains the lack of crystal growth. Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.
OBSIDIAN PROPERTIES:
Category: Volcanic glass
Color: Deep black or blackish green
Fracture: Conchoidal
Mohs scale hardness: 5–6
Luster: Vitreous
Specific gravity: 2.4
Optical properties: Translucent
Other characteristics: Texture: Smooth; glassy
Courtesy of Wikipedia