Obsidian is natural glass that was formerly molten rock connected with a volcano. This volcanic glass has an approximately whole deficiency of considerable mineral crystals surrounded by the glass matrix. When I say "crystals," don't envisage those fine-looking pointed prisms of quartz bring into geodes. All rocks consists of mixtures of a variety of crystalline minerals. When crystallization happens, the atoms that embrace a mineral turns out to be approved in customary, statistical patterns that are exclusive to the explicit mineral.
Crystal faces structure only where there is sufficient unwrap breathing space in the rock throng to consent to the normal arithmetical forms of the crystals to expand as liberated faces. Granite is unruffled completely of inter-grown crystals of quartz, feldspar, mica and additional minerals. These comparatively huge mineral crystals (effortlessly noticeable to the stripped eye) give granite a coarse fracture exterior. Like every glass and some supplementary types of unsurprisingly occurring rocks, obsidian breaks with a distinguishing "conchoidal" rupture. This silky, curved brand of rupture surface happens because of the mere absence of mineral crystals in the glass.
The intersections of conchoidal rupture surfaces can be sharper than a straight razor. This had palpable advantages for our Stone Age ancestors, who utilized obsidian expansively for tool manufacturing. Obsidian contains of about 70 percent or additional non-crystallized silica (silicon dioxide). It is chemically analogous to granite and rhyolite, which also were in the beginning molten. For the reason that obsidian is not contained of mineral crystals, in principle obsidian is not a factual "rock." It is actually a frozen fluid with minor influences of infinitesimal mineral crystals and rock impurities. Obsidian is comparatively squashy with an archetypal hardness of 5 to 5.5 on the mineral rigidity scale. In similarity, quartz (crystallized silicon dioxide) has a hardness reading of 7.0.
Obsidian occurs only at places where geologic processes generate volcanoes and where the chemical concerto of the molten rock is wealthy in silica. Obsidian-bearing volcanoes are characteristically positioned in or close to areas of crystal instability or mountain building.