Obsidian

A natural glass, formed when lava cools too quickly to form crystals, such as when lava erupts in small amounts and cools in hours rather than days.

Similar in structure, but harder than glass, obsidian has been known and worked since neolithic times, utilised for tools and weapons.

For those who know the movie "Stardust" (2007) the knife used by Michelle Pfieffer is fashioned of obsidian.

The reddish tones of mahogany obsidian are caused by the molten rhyolite magma mixing with iron oxide. Snowflake obsidian, which contains patched of christobalite (the “snowflakes” can be caused by devitrification when moisture alters the silica in the obsidian), is said to be a protection stone, balancing body,mind and spirit. Rainbow obsidian forms when tiny gas bubbles are trapped in the glassy rock.