|The obsidian in the photo is displayed at Stephen Hui Geological Museum, Hong Kong.|
Credit: Stephen Hui Geological Museum
Green Obsidian – Volcanic glass
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as a mineraloid.
Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian. Rarely, volcanic glasses are found with a composition similar to basalt and gabbro. These glassy rocks are named "tachylyte."
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. This can be caused by tiny microscopic inclusions of other minerals (feldspar, amphibole, biotite, quartz) called microlites (not the mineral). They can be observed under a microscope as thin rectangular, sometimes equant crystals that can sometimes form bands and give obsidian its characteristic colour.
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.
|Green Obsidian from Davis Creek in Northern California, USA|