By the nature of Genesis
In general you can divide glass into two groups: natural glass and artificial glass. While artificial glass is produced by the melting of several raw materials, natural glass is produced by processes in nature. The best known of such processes has to be the formation of obsidian and pumice. These natural types of glass result from volcanic eruptions of lava. Even a few millennia ago such natural glass was being used by humans for various purposes. Other natural glasses include tektites (meteorite impact) and fulgurites (lightning impact).
By the nature of „chemistry“