Because obsidian is metastable at the Earth’s surface, over time the glass becomes fine-grained mineral crystals, no obsidian has been found that is older than the Cretaceous period. Crystalline rocks with a similar composition include granite and rhyolite. Obsidian consists mainly of silicon dioxide, usually 70% or more. Though obsidian is usually dark in color, similar to mafic rocks such as basalt, obsidian’s composition is extremely felsic. It is sometimes classified as a mineraloid. In addition, its composition is too variable to be classified as a mineral.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline. Intrusive formation of obsidian may occur when felsic lava cools along the edges of a dike. Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air. Obsidian is formed from quickly cooled lava, which is the parent material. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades. Obsidian is hard, brittle, and amorphous. The inhibition of atomic diffusion through this highly viscous lava explains the lack of 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. Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. Buy natural starry night obsidian in our shop Obsidian volcanic glass Starry night obsidian, also named Firecracker obsidian or flower obsidian.Īn intriguing combination of black obsidian with splashes of coral, cream, pink and white snowflakes throughout.
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