What are glass bottles made of?
Glass, of course.
But all glass isnt the same, just like not all plastic is the same.
Most container glass is soda-lime glass. Soda-lime glass is mostly silica dioxide (~74%), sodium oxide (~13%), and calcium oxide (~10-11%).
The other 2-3% consists of a variety of chemicals such as alumima-oxide, magnesium oxide, potassium oxide, iron oxide, titanium dioxide, sulfur trioxide, and various other chemicals.
The various chemical mixes give various results. For instance, small amounts of gold in the mix can make the glass purple.
The common answer that glass is mostly sand is partly right, but it is important to know what the sand is made of. Much sand is mostly silica dioxide, or quartz, but beach sand usually consists of a variety of materials. Some sand, like that found on many white sand beaches, is mostly broken down corals and shellfish. That sand is primarily aragonite, also known as calcium carbonate.