Collection: Opal

Opal is a mineral with a very beautiful optical effect (due to its amorphous nature, it is technically a mineraloid, though the International Mineralogical Association classifies it as a formal mineral).

The iridescent optical effect is called play-of-color, and opals exhibiting this are called precious opals, while those that do not are called common opals. Generally, opals contain a high amount of water, so care is needed in handling them. In this regard, black opals from Australia are stable, while opals from Welo, Ethiopia, tend to degrade more easily. Additionally, there is hyalite (uranium-bearing), which, although lacking play-of-color, is colorless and transparent and tends to show a greenish hue.

Name: Opal
Japanese Name: Tanpakuseki
Chemical Formula: SiO₂・nH₂O
Crystal System: Amorphous (not crystalline)
Specific Gravity: 1.9–2.3
Mohs Hardness: 1.9–2.3
Cleavage: None, but inherently brittle
Luster: Vitreous to Resinous
Transparency: Transparent to Opaque
Refractive Index: 1.37–1.52 (predominantly 1.44–1.46)
Birefringence: None
Color: Colorless, white, red, black, grey + play-of-color
Dispersion: 0.020