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Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age.This breakdown of obsidian is accelerated by the presence of water.Two colours can also be found at a single obsidian where black and brown are the most common to occur being associated at a single rock body.Rarely obsidian can be of iridescent where are called as rainbow obsidian, golden obsidian or silver obsidian.Obsidian is a glass where lava flow solidifies rapidly forming no crystalline structure but it is also not stable at this form and with time begins forming crystals not at the whole rock body but at different parts of the obsidian.It is not at a regular time interval so great time is required to get obsidian crystalline form.Rare colours of obsidian can also be blue, red, orange or yellow.The colour of obsidian is thought to be from the trace elements.
Obsidian is the rock formed as a result of quickly cooled lava, which is the parent material.
Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerisation degree of the lava.
The inhibition of atomic diffusion through this highly viscous and polymerised lava explains the lack of crystal growth.
Pumice, scoria, and tachylyte are other volcanic glasses formed by rapid cooling.
Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt.
Iron and other transition elements may give the obsidian a dark brown to black colour. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian).