Glass is one of the noncrystalline (amorphous) forms of quartz (SiO2). Quartz is crystalline SiO2 (structure shown in figure (a) below), while fused silica is SiO2 which is amorphous SiO2 without impurities ( the structure is shown in figure (b) below).
In practice, impurities (such as sodium shown in the figure below) are added to the glass to lower the melting temperature and the viscosity of the glass to make it easier to work the glass at lower temperatures.
Glass's amorphous structure breaks up the band structure of SiO2 such that there are no electronic states that electrons can jump to by absorbing visible light in glass. Here is a TED-Ed video by Mark Miodownik (the host of the Secret Life of Materials videos) to explain this in more detail. In the next sections, we are going to discuss why glass is brittle and how glass is being engineered not to be so brittle.
Please watch the following short TED Ed video, Why is Glass Transparent?, before proceeding to the next section on the Glass Age.