|Natural Green Obsidian Volcanic. Photo: honestcrysta|
Obsidian is a naturally occurring volcanic glass. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Because of their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas we see on the island of Hawaii.
Green Obsidian is one of obsidian rocks that contain impurities. Iron and other elements of transition can give a dark brown to black color to the obsidian. Most black obsidians contain magnetite, an iron oxide, nanoinclusions, the composition of obsidian is extremely felsic.
In the case of dark green obsidian the colouration is caused varying amounts of Fe and Mg. The green coloration can also be achieved as some gas bubbles (vescicles) remained in the lava flow during crystallization.
Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. 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.
Green Obsidian is also used for ornamental purposes and as a gemstone. It presents a different appearance depending on how it is cut: in one direction it is jet black, while in another it is glistening gray. "Apache tears" are small rounded obsidian nuggets often embedded within a grayish-white perlite matrix.
|Green Obsidian from Davis Creek in Northern California, USA|
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