Opal

Overview

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O), containing 3 to 21 percent water. Unlike crystalline gemstones, opal has no crystal structure. Its famous play-of-colour (a shifting display of spectral colours) results from light diffracting through a regular array of microscopic silica spheres within the stone.

Physical Properties

Mohs hardness: 5.5 to 6.5. Refractive index: 1.370 to 1.520. Specific gravity: 1.98 to 2.50. Opal is relatively fragile compared to other popular gemstones. Its water content makes it susceptible to crazing (surface cracking) if it dries out or experiences sudden temperature changes.

Types

Black opal: The most valuable variety, with a dark body colour that makes the play-of-colour appear more vivid. Lightning Ridge (Australia) is the premier source. White opal: Light body colour with play-of-colour. More common and affordable. Fire opal: A transparent to translucent orange to red opal, with or without play-of-colour. Mexico is the primary source. Boulder opal: Opal forming in ironstone matrix, cut with the host rock. Produces unique patterns. Crystal opal: Transparent with vivid play-of-colour. Ethiopian opal has emerged as a significant source of this type.

Major Sources

Australia produces over 90% of the world's opal. Lightning Ridge (black opal), Coober Pedy (white opal), and Queensland (boulder opal) are the key regions. Ethiopia has become the second most important source, particularly for crystal and white opal. Mexico produces fire opal. Brazil, Honduras, and Indonesia also yield opal.

Care Requirements

Opal requires more care than most gemstones. Store in a slightly humid environment (a sealed bag with a damp cotton ball works). Avoid extreme heat, direct prolonged sunlight, and ultrasonic cleaners. Clean only with a damp cloth. Solid opals are more stable than doublets or triplets (layered opal products).