Although most people think of glass as a man-made material, it is found in many forms in the natural world. A range of glasses are found in—and formed by—nature.

Volcanoes spew molten rock, lightning strikes desert and beach sands, meteorites pound the earth, and sea sponges and microscopic organisms inhabit the waters. All of these natural phenomena are related to glass.

What is glass? Scientists tell us that glass is a state of matter rather than a single material. It is formed when a molten material cools so rapidly that there is not enough time for the material’s crystalline structure to re-form.

Solids have atoms and molecules arranged in perfectly ordered, lattice-like structures. In liquids and gases, atoms and molecules are free to move about in a random way, which is why they can flow. In glass, the atoms and molecules are held rigidly in place so that they cannot flow. But they have not had time to arrange themselves in the perfectly ordered lattice that nature would have liked. Neither a solid nor a liquid, glass is often called a rigid liquid. It is unlike any other material on earth.

In nature, glasses are formed when sand and/or rocks, often high in silica, are heated to high temperatures and then cooled rapidly. The intense heat and force of meteoritic impacts on the earth and atmospheric explosions, millions of years ago, created the natural glasses that we know as tektites and Libyan Desert Glass. Libyan Desert Glass [such as 2000.7.1] is found in the Sahara’s Great Sand Sea, which spreads across the border of Libya and Egypt. The large silica glass field there is believed to have resulted either from a meteoritic impact or from a comet exploding in the earth’s atmosphere.

Tektites [such as 2000.7.4] were created by meteoritic impacts. These impacts melted the meteor and the surrounding terrestrial rock, creating the spherical forms, comma shapes, drips, and splashes characteristic of tektites. Tektites are formed at a much higher temperature than volcanic glass (obsidian) and under tremendous pressure.

Natural glass may also be formed by explosions on earth. Volcanic glass, known as obsidian [63.7.12], is created when lava cools rapidly, becoming rock in a glassy state, just as boiled hard sugar candies are sugar in a glassy state.

A type of obsidian found in Arizona is called an “Apache tear” [70.7.4]. The name "Apache tear" comes from an Apache legend. The stones represent the tears shed by the families of a band of 75 Apache men killed during a retaliatory raid by the U.S. military in the 1870s. The 25 men who survived the first strike chose to die by riding their horses over the edge of Apache Leap Mountain.

Natural glass may be formed by weather. Glass is made when lightning strikes sand, melting the sand to form a glassy tube. These glassy tubes are called fulgurites [such as 2009.7.2], and they are found everywhere. Sometimes, fulgurites are called petrified lightning. Fulgurites vary in length from a few inches to several feet, and they break easily. They occur naturally from lightning strikes and they can be man-made. The longest recorded fulgurite, at Camp Blanding in Florida, measures 17 feet.

One type of man-made glass is similar to glass formed by meteoritic impacts and by lightning strikes. Trinitite was unintentionally created by the United States Army on July 16, 1945, during the testing of the Trinity atomic bomb in White Sands, New Mexico. During the explosion, sand was sucked up into the nuclear fireball, falling back to the ground as molten glass. See 2006.4.237.

Natural glasses occur above and below the earth’s surface. Sea sponges are among the oldest known multicellular animals. The spicules (needlelike structures) that form the sponge’s skeleton are made of such materials as silica, carbonates, and protein fibers.

Glass sponges, which live in the deep ocean, have skeletons made of silica spicules. A scientific study of its substructure and refractive properties found that the sponge’s silica spicules transmit light in a similar way to the optical fibers used in telecommunications. See 2005.7.26.

Microscopic organisms, such as diatoms (algae) and radiolarians (protozoans) also have siliceous (silica) skeletons. Ernst Haeckel, the 19th-century German naturalist and philosopher documented these organisms in his illustrations.

Diatoms are an important food resource for marine and freshwater microorganisms, and they are a major source of atmospheric oxygen. Not all diatoms float freely in the water. Many cling to aquatic plants, invertebrates, and even turtles. Whales may carry dense growths of diatoms on their skin. Some diatoms are found in the soil or on moist mosses.

Radiolarians are amoeboid protozoa that are found as zooplankton throughout the ocean. Because they appear as early as the Paleozoic era (500 million years ago), they are highly diverse. Their siliceous skeletons act as important diagnostic fossils, helping scientists understand the history of life on the planet.

Published on August 31, 2012