What type of solid is silicon dioxide?

Also called common silica, it may exist in crystalline form as quartz, or as an amorphous solid (or supercooled liquid) called glass. Physically it is quite durable, hard and resists high temperatures but exhibits a brittle fracture. Chemically it is covalently bonded, fairly inert, resists strong alkalis and most acids except Hydrofluoric Acid and only very very sparingly soluble in water.