Glass occurs in three forms in nature. Obsidian, a volcanic glass, was named for Obsius, the person who, according to Pliny the Elder, first discovered the material in Ethiopia.  Moldavite, usually classified as a member of the tektite group, was named after the area along the Moldau (Vltava) River in the Czech Republic where it was first found and was probably formed in meteorite impacts that melted existing rock.  Small jagged pieces of glass, called fulgurites, are sometimes found in sandy or rocky dry areas and are thought to be associated with lightning strikes.

Glass was manufactured in the ancient world by melting a combination of an alkali (potash or soda) and silica (raw materials such as quartz pebbles and sand), a formula that is basically the same today. Although known to many civilizations since very remote times and used for ritual objects, it was the Romans who first used glass in everyday items. The chemical composition of ancient glass objects is similar to that of common modern era glass. 

The hardness of glass is average - between 5 and 5.5 on the ten-point Moh's scale (Diamond is 10 and Talc is 1).

Since ancient times, it has been known that glass can be manufactured in almost any color by adding metal salts. Adding cobalt oxide produces blue color, for example, while green glass derives its color from compounds of either copper or iron.