Obsidian

What is Obsidian?

Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo on right).


Where Does Obsidian Form?
Obsidian is usually an extrusive rock - one that solidifies above Earth's surface. However, it can form in a variety of cooling environments:

    along the edges of a lava flow (extrusive)
    around the edges of a sill or a dike (intrusive)
    where lava contacs water (extrusive)
    where lava cools while airborne (extrusive)

source: geology.com