Silica <> glass
Silica or quartz is not (often) a true glass. Fused silica is used in optics, but due to the high temperatures needed for glass transition it is limited in its use. Typical glass combines silica (about 75%) with sodium and calcium oxides. So silica is the main constituent in many cases, but not the whole story. Most silica typically has a proper crystalline structure, and is not glass-like in many properties (e.g. thermal conductivity of glass is typically closer to that of certain liquids than that of crystalline silica or carborundum (alumina)).