Opal /ˈoʊpəl/ (Latin opalus, Greek opallios, from Sanskrit “fell” – precious stone) is a water-containing colloidal silicon oxide of globular structure.
Opal is very fragile and cracks when dehydrated. Hardness 5-6.5. Density 1850-2300 kg/m3. The luster is glassy, sometimes pearlescent or waxy.