Obsidian is an extrusive igneous rock that is composed mainly of the minerals feldspar and quartz. It has been used as a natural form of glass for its cutting properties since prehistoric times - being known as volcanic glass - and today is used as a gemstone.
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A perlite is an obsidian, or other vitreous rock with a concentric structure and which is expansible by heating.
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Pitchstone is a dark, glassy igneous rock produced by the rapid cooling of lava. It is composed essentially of feldspar and quartz and is a natural glass, the glass forming because the rock cools too quickly for the minerals to form into crystals. Pitchstone is closely related to obsidian, but where pitchstone has cooled more slowly than obsidian, it is less shiny and shows signs of devitrification.
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Pumice (formerly known as barber's stone) is a light igneous volcanic rock used as a scourer, abrasive, polisher and in the manufacture of soundproofing tiles. Pumice can occur in any type of lava, but is more common in rhyolite and trachyte than in basalt. Pumice is formed from molten lava that cools so quickly that it doesn't have time to form either crystals or glass. Pumice may be heated until it melts, and will then form obsidian. Pumice is full of tiny vesicles which are the solidified remains of bubbles that were in the molten rock. These vesicles may pumice very light and porous - pumice will float and absorb water until saturated.
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Tektites are small, irregularly shaped glassy nodules that have no crystal structure and are similar to obsidian. Tektites are thought to be from meteorites, but their origin is not proven, some scientists maintaining that they are terrestrial and produced as a result of meteorite impacts, others believing them to be meteorite material.
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