Glass

An amorphous, homogeneous material with a random liquid-like structure generally formed due to rapid cooling.
Glasses can be natural (such as obsidian) or artificial (e.g., in slags). Artificial glasses are predominantly composed of silica with a few impurities.

Crystals in igneous rocks may contain tiny inclusions of frozen melt, i.e., glass.

In gemmology, man-made glasses have been used as gems since the discovery of glass, and special glass varieties, such as lead glass, containing additives have been used to simulate more valuable gemstones, such as diamond.