Obsidian SiO2::MgO, Fe3O4 - 4.DA.05 (Strunz) (from San Pedro de Atacama, Chile and Snowflake Vari
Obsidian SiO2::MgO, Fe3O4 - 4.DA.05 (Strunz) (from San Pedro de Atacama, Chile and Snowflake Variety from Arizona, USA) Obsidian is not a mineral, but a volcanic glass formed by the rapid cooling of volcanic lava. The variety “snowflake” is charcaterized by crystals of cristobalite, a polymorph of quartz. In prehistoric, obsidian was used to make weapons and spearheads. Presents pleochroism (black and grey) -- source link
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