See also: Obsidian: Everything About Volcanic Glass California Rainbow Obsidian Is a Natural Wonder Green Obsidian from Davis Creek in Northern California, USA "Apache tears" are small rounded obsidian nuggets often embedded within a grayish-white perlite matrix. 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. Green Obsidian is also used for ornamental purposes and as a gemstone. 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. Crystalline rocks with obsidian's composition include granite and rhyolite. Obsidian consists mainly of SiO 2 (silicon dioxide), usually 70% or more. Over the millennia, its seen the exchange of goods such as jade, copper, basketry, food, hides, obsidian and the highly valued commodity the trail was named for: the nutritious oil or 'grease'. The green coloration can also be achieved as some gas bubbles (vescicles) remained in the lava flow during crystallization. In the case of dark green obsidian the colouration is caused varying amounts of Fe and Mg. They can be observed under a microscope as thin rectangular, sometimes equant crystals that can sometimes form bands and give obsidian its characteristic colour. This can be caused by tiny microscopic inclusions of other minerals (feldspar, amphibole, biotite, quartz) called microlites (not the mineral). Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Most black obsidians contain magnetite, an iron oxide, nanoinclusions, the composition of obsidian is extremely felsic. Iron and other elements of transition can give a dark brown to black color to the obsidian. Green Obsidian is one of obsidian rocks that contain impurities. 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. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Obsidian is a naturally occurring volcanic glass. It is a rare and valuable gemstone that is found in only a few places in the world, including Mexico, Guatemala, and the United States. Green obsidian is a type of natural glass that gets its name from its green color.
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