Minerals are added to silica to change the color.
Cobalt oxide makes glass turn blue.
Adding iron gives glass an aquamarine cast. Cobalt and nickel together make the glass colorless. Phospherous makes glass opalescent. Glass can be made from two to six times stronger, as well as less brittle and more resistant to fracture from temperature extremes, by reheating it and then quickly chilling it. Manufaturers make glass stronger by mixing more iron into the silica.