What is Glass?
According to the Columbia Electronic Encyclopedia, glass is a:

Hard substance, usually brittle and transparent, composed chiefly of silicates and an alkali fused at high temperature.

Properties of Glass
Glass is made of a mixture of silica that comes from sand. The properties of glass vary by type. For example, lead is added for brilliance and weight; boron is added to make the glass resist changes in temperature or electricity; barium increases the refraction of light; metallic oxides give the glass color.

Source:
The Columbia Electronic Encyclopedia, 6th ed. Copyright 2005, Columbia University Press. www.infoplease.com