| • | In its pure form, glass is a transparent, relatively strong, hard-wearing, essentially inert, and biologically inactive material which can be formed with very smooth and impervious surfaces. These desirable properties lead to a great many uses of glass. Glass is, however, brittle and will break into sharp shards. These properties can be modified, or even changed entirely, with the addition of other compounds or heat treatment.
Common glass is about 70% amorphous silicon dioxide (SiO2), which is the same chemical compound found in quartz, or in its polycrystalline form, sand.
Source: [wikipedia: glass]
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