Obsidian is a volcanic rock that is formed by lava flow which solidifies so quickly that its atom do not have time to arrange themselves in a crystalline structural form.
Colour of obsidianObsidian mainly forms of black colour however this colour is not the only one but is the most common colour. It can also be of different colours like brown, tan or green. Rare colours of obsidian can also be blue, red, orange or yellow. The colour of obsidian is thought to be from the trace elements. 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.
Stability of 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.
Composition of obsidian is same as that of rhyolite and granite so they can be associated with obsidian and are rarely formed with basalt and gabbro.
Volcanic glasses other than obsidian
Other volcanic glasses also occur in igneous rock that are named as pumice and scoria but they differ from obsidian in just a way of vesicular structure by escaping gases.
Occurence of obsidian
Obsidian occurs at variety of cooling environments
- along edges of lava flow
- along edges of volcanic dome
- around edges of sill and dykes
- where lava contacts water
- where lava cools while airborne
Obsidian is used as a cutting tool because of its conchoidal fracturing where it breaks into thin sheets and have sharp edges. In stone age obsidian was used as a cutting tool for making any sharp tool and it is still used as a cutting tool in modern surgeries. It is also used as a jewellery by making beads of obsidian.