Obsidian - is a naturally occurring volcanic glass formed as an extrusive igneous rock. Obsidian has a low water content when fresh, typically less than 1% water by weight. It becomes progressively hydrated when exposed to groundwater, forming perlite. Like other volcanic glasses, obsidian is a lava that has cooled too quickly for the contained minerals to crystallize. It is homogeneous in texture volcanic glass.
It is magma, effusive (extrusive) material.
In chemical composition it is rich in silica (75%> SiO2 ≥ 65%) and similar to granite
The color is black, gray, rust-coloured; the position of bars indicates the direction of lava flow.
The structure is glassy (structureless), dense.
Texture of Obsidian is massive or frothy. Formation density is 2500—2600 kg/m³. It is spalt, has lower abrasion resistance. Having low water content when newly formed typically less than 1% water by weight, has a smooth, shiny texture of a thick piece of glass.
Obsidian is a member of newest rock formation. As distinct from quartz (morion, cairngorm) has amorphous structure.
Obsidian often occurs together with liparite and liparite porphyrite. It forms small flows: because of persilicic composition lava flows are extremely viscous.
Due to an ability of obsidian powder in combination with slaked lime to harden under water, Obsidian is used as pozzolan for portland cement. It is also used as an additive for lime, a raw material for the manufacture of dark glass, heat retention lagging. It is ornamental stone. Obsidian is used by some surgeons for scalpel blades, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, the cutting edge of the blade being only about 3 nanometers thick.
Obsidian plaid an important part in the history of mankind: it is used for producing of tools, weapons, and jewelries; from ancient times peoples it used in glyptic and ornamental sculpture. Maya, who can't smelt iron, used obsidian to make cutters.