What makes transparent glass transparent?
Glass is an amorphous solid -- the molecules are not arranged in an orderly fashion. That makes glass unlike most solids.
When molecules are neatly organized (like puzzles pieces that fit together) they create a barrier to light. When the molecules are more or less jumbled together randomly, as they are with transparent glass, they do not fit as well and there are (lots) of gaps through which light can pass.
When a large percentage of those unstructured molecules are silica then even more light passes through the material. That's because silica molecules tend to transmit photons which are the building blocks of light.
If you are a glass artist and have ever battled devitrification then you've seen what happens when the glass molecules are given the opportunity to organize: crystals form and light is blocked.
