Although most people think of glass as a man-made material, it is found in many forms in the natural world. 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 things—and even lunar soils—are materially related to the man-made glass that we use every day.
Scientists tell us that glass is a state of matter rather than a single material. Glass is created when a molten material cools so rapidly that there is not enough time for a crystalline structure to form. To the scientist, crystals are materials that have their atoms arranged in perfectly ordered, lattice-like structures. In liquids, atoms and molecules are free to move about in a random way—which is why liquids can flow. In a glass, the atoms are held rigidly in place so it cannot flow. But they have not had time to arrange themselves in a perfectly ordered lattice. Neither a solid nor a liquid, glass is often called a rigid liquid.
In nature, glasses are formed when sand and/or rocks, often high in silica, are heated to high temperatures and then cooled rapidly. The Glass in Nature display shows specimens of glass made in nature. Obsidian or volcanic glass, for example, is molten rock that has quickly cooled, becoming rock in a glassy state. Tektites and Libyan Desert Glass are other forms of glassy rock created by the intense heat and force of meteoritic impacts on the earth millions of years ago. Fulgurites, which are made when lightning strikes sand, are brittle tubes of melted sand. Some marine creatures, such as microscopic algae and sea sponges, have siliceous (silica) skeletons, which are also a form of natural glass.