Mineral Type - Opal
Opal is classified into four types:
Opal-CT Cristobalite-Tridymite;
Opal-C Cristobalite;
Opal-AG Amorphous-Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal); and
Opal-AN Amorphous-Network (found as Hyalite)
Transitions between opal-AG, opal-CT and opal-C are common.
Studies at low temperature show that water molecules way be organized into ice-like structure, which includes the cubic ice modification (Eckert et al., 2015).
Transparency: Transparent, Translucent
Colour: Colourless, white, yellow, red, orange, green, brown, black, blue
Streak: White
Hardness: 5½ - 6½ on Mohs scale.
Tenacity: Brittle
Cleavage: None Observed
Fracture: Irregular/Uneven, Splintery, Conchoidal
Density: 1.9 - 2.3 g/cm3 (Measured)
Common Impurities: H, O, Si
Opal-CT Cristobalite-Tridymite;
Opal-C Cristobalite;
Opal-AG Amorphous-Gel (closely packed amorphous silica spheres form a diffraction grating to create Precious Opal); and
Opal-AN Amorphous-Network (found as Hyalite)
Transitions between opal-AG, opal-CT and opal-C are common.
Studies at low temperature show that water molecules way be organized into ice-like structure, which includes the cubic ice modification (Eckert et al., 2015).
Name
The origin of the name is uncertain. It may be from the Sanskrit "upala", meaning "stone" or "precious stone" or from opalus, the ancient Latin name for the gem (Pliny the elder, 75-79). Pliny may have also referred to the gem as paederos, but a modern commentary by Kostov (2008) questions if that name was actually applied to the opal of modern sense.Physical Properties
Lustre: Vitreous, Waxy, Greasy, DullTransparency: Transparent, Translucent
Colour: Colourless, white, yellow, red, orange, green, brown, black, blue
Streak: White
Hardness: 5½ - 6½ on Mohs scale.
Tenacity: Brittle
Cleavage: None Observed
Fracture: Irregular/Uneven, Splintery, Conchoidal
Density: 1.9 - 2.3 g/cm3 (Measured)
Chemical Properties
Formula: SiO2 · nH2OCommon Impurities: H, O, Si