Facts

Volcanic glass: Obsidian forms from rapidly cooled lava, resulting in a natural glass without a crystal structure.

Sharp edges: Obsidian can be fractured to create extremely sharp edges, which is why it was used for blades and tools by ancient civilisations.

Ancient uses: Obsidian has been used for arrowheads, knives, and mirrors since prehistoric times, with a history of use in civilisations like the Aztecs and Mayans.

Found near volcanoes: Major deposits of obsidian are found near volcanic regions, such as Mexico, the United States, and Italy.

Science

Mineral: Volcanic glass (not a true mineral)
Crystal System: Amorphous
Chemistry: SiO₂ (with impurities)
Colour: Typically black, but also brown and multicoloured
Refractive Index: 1.45 to 1.55
Birefringence: None
Specific Gravity: 2.3 to 2.6
Mohs Hardness: 5 to 5.5

Similar Materials

Jet: Often mistaken for obsidian due to its black colour, but jet is a fossilised wood with a lower hardness and lighter feel.

Onyx: A black variety of chalcedony that may resemble obsidian but has a crystalline structure and is typically banded.

Tourmaline: Black tourmaline may be confused with obsidian due to similar colouring, but it has a much higher hardness and a crystal structure.

Treatments

Obsidian is typically untreated, as its natural glassy appearance is highly valued.

Synthetics

There are no widely produced synthetic versions of obsidian.

Imitations

Coloured glass can be used to imitate obsidian, especially black glass. However, glass imitations lack the volcanic origin and the distinctive natural qualities of real obsidian and variation in types.