This is the black obsidian pebble with irisation. Dimensions: 2.7 x 1.7 x 0.9″ (6.9 x 4.2 x 2.4cm). Weight – 3.2oz (92 grams). Origin: Russia, Siberia.
Some information about obsidian:
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
Obsidian 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 used to manufacture cutting and piercing tools, and experimentally as surgical scalpel blades.