An amorphous solid is any material which does not have its molecules arranged in a lattice, or crystalline structure. Amorphous solids are relatively rare, making up only 10 percent of solids in the world. The most well known example of an amorphous solid is glass, and in fact, these solids are sometimes generally termed glasses.
The three states of matter — solid, liquid, and gas — occur because of the varying degrees of movement of the molecules that make up the substances. Molecules and atoms in gases have a wide range of movement, spreading out because the molecules are weakly linked together. Liquids have a more restricted range of movement, yet their molecules still move freely within those confines, changing position. This freedom is what gives liquids their lack of permanent shape.
Molecules in solids, on the other hand, are bound together with no real freedom of movement. The molecules still shift, however, oscillating in their bonds. The oscillating movement is what allows solids to heat up. The faster the molecules shake in their bonds, the hotter the object gets.
In crystalline solids, which make up 90 percent of the solids in the world, molecules are bound in an ordered patterned. This ordered pattern repeats exactly throughout the entire structure, creating a lattice of molecules. Conversely, an amorphous solid may have a repeating pattern for small portions of its make-up, but not for its entirety.
Amorphous solids may be made natural or man-made. Lightning striking sand will naturally cause glass to form at its strike point. Commercial glass, however, is man-made, using a process that creates the same conditions as lightning, but in a controlled environment. In addition to glass, one of the most widely used amorphous solids is plastic. Plastic is made from polymers, long strings of molecules purposefully chained together.
Additionally, amorphous solids can be made out of crystalline solids. For example, glass is made from quartz sand, a crystalline solid. The sand is heated to extreme temperatures, which effectively melts it, and then cooled quickly, or supercooled, so the molecules do not have time to arrange themselves back into a lattice form. This supercooling results in a molecular arrangement of the glass similar to if someone had taken a snapshot of the liquid.
Since amorphous solids' structures appear similar to liquids, they are sometimes called supercooled liquids. This similarity is also why the myth developed that glass is actually a slowly-moving liquid. Additionally, unlike a crystalline solid, an amorphous solid does not immediately liquefy when exposed to heat, but instead grows increasingly soft.