Silica, or silicon dioxide (SiO2), may exist in several crystalline polymorphic or in amorphous forms. The surface of such amorphous silica contains numerous silanol groups, can contain adsorbed water molecules, or can be anhydrous (except for a monomolecular layer of water). It is due to the capability of silica materials to be made porous, with a large surface area covered with reactive groups (silanol, Si-OH) and at the same time to have a high rigidity and resilience to crashing.