The Composition and Properties of Glass

Close-up of Moldavite Precious Gemstone
A precious moldavite gemstone in its non-crystalline, glass form. Ron Evans / Getty Images

When you hear the term "glass" you may think of window glass or a drinking glass. However, there are many other types of glass.

Glass is the name given to any amorphous (non-crystalline) solid that displays a glass transition near its melting point. This is related to the glass transition temperature, which is the temperature where an amorphous solid becomes soft near its melting point or a liquid becomes brittle near its freezing point.

Glass is a type of matter. Sometimes the term glass is restricted to inorganic compounds, but more often now a glass may be an organic polymer or plastic or even an aqueous solution.

Silicon Dioxide and Glass

The glass you encounter most often is silicate glass, which consists mainly of silica or silicon dioxide, SiO2. This is the type of glass you find in windows and drinking glasses. The crystalline form of this mineral is quartz. When the solid material is non-crystalline, it is a glass.

You can make glass by melting silica-based sand. Natural forms of silicate glass also exist. Impurities or additional elements and compounds added to the silicate change the color and other properties of the glass.

Glass Examples

Several types of glass occur in nature:

  • Obsidian (volcanic silicate glass)
  • Fulgurites (sand that has been vitrified by a lightning strike)
  • Moldavite - green natural glass likely resulting from meteorite impacts

Man-made glass includes:

  • Borosilicate glass (e.g., Pyrex, Kimax)
  • Isinglass
  • Soda-lime glass
  • Trinitite (radioactive glass formed by heating of the desert floor by the Trinity nuclear test)
  • Fused quartz
  • Fluoro-aluminate
  • Tellurium dioxide
  • Polystyrene
  • Rubber for tires
  • Polyvinyl acetate (PVA)
  • Polypropylene
  • Polycarbonate
  • Some aqueous solutions
  • Amorphous metals and alloys