The most common impurities found in natural silica are iron sesquioxide (Fe2O3), alumina (Al2O3), and calcium compounds. Ferric ... Yellow glass is usually made with cadmium sulfide, and red or pink glass usually contains selenium, although some ruby-colored ... Colored glass is made by adding metallic compounds to the basic sand/soda/lime mixture. For instance, red glass is made by ... The Egyptians of 1500 b.c.e. knew that they could make brightly colored glasses by adding certain metals (or their compounds) ...