Volcanic glass is that which is naturally formed when high viscosity molten lava cools. The glass is created because this lava contains less water and more crystal material. Its chemical composition is similar to that of granite, which is composed of alkali feldspar and quartz. The materials in the magma, the water content, and speed of cooling all contribute to a variety of natural glasses.
Magma can crystallize while cooling. The higher the viscosity of the lava, the more likely it will be to turn into glass. Igneous rock is also produced from the cooling of lava, and often contains crystal particles. Much of Earth’s crust is formed from igneous rock material.
Rapid vaporization of rhyolitic or felsic lava produces the volcanic glass called obsidian. While obsidian can be formed with a variety of crystals, it has high silica content. This glass is usually black in color, but the presence of hematite can create brown, green, and red shades as well. Mount Helka in Iceland is one of the volcanoes known for producing obsidian. While it is a glass, it is also considered a natural gemstone.
Tachylyte is an igneous rock that is glassy in appearance. It is low in silica and formed when non-acidic lava cools very quickly. This glassy rock is commonly black with thin brown sections. Magnetite granules can be found in tachylyte as well. This type of volcanic glass can be found in the Hawaiian Islands and the Inner Hebrides of Scotland.
Pele’s tears are small pieces of lava that have cooled quickly. These pieces become teardrop-shaped and black in color. Often found in Hawaii, the glass was named for the Hawaiian goddess of volcanoes. Apache tears are another type of black volcanic glass. These tear-shaped pieces of obsidian can be translucent when held to light, and sometimes include shades of brown or red.
Pumicite is volcanic ash and is very abrasive. It contains very small particles of volcanic glass. The ash can be used for polishing and grinding. Perlite is a volcanic rock that can be used as an insulator in building construction. It can also be found in potting soils, where it is used to improve drainage and aeration.
Spontaneous changes, devitrification, occur in volcanic glass that can make it become stone-like in appearance. These changes frequently begin in areas where larger crystals or cracks are present in the glass. Over time, the glass will become crystals of feldspar, tridymite, and quartz. This process occurs over a long period of time — up to millions of years — but most volcanic glass found is less than 65 million years old.