Opal is a hydrated silica material with a hardness of 5.5 to 6.5. It is composed of submicroscopic silica spheres that are bonded together with water and additional silica. Sometimes the water in Opal can evaporate, creating cracks in the stone. Some, but not all Opals exhibit the play of colors known as ‘fire’. This is caused by the Opal’s minute silica spheres being packed in a formation regular enough to cause light diffraction. The name Opal is derived from the Latin opalus and the Sanskrit upala, meaning ‘precious stone’.