Opal

Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

Formation & Geology

Australian opal is often found in sedimentary rocks from the Cretaceous age. It forms when silica-rich water seeps into cracks and voids in the earth. As the water evaporates, it leaves behind a silica deposit. Over long periods of time, this cycle repeats, forming opal. The unique play of color in precious opal is caused by the diffraction of light by silica spheres packed in a regular arrangement.

Physical Characteristics

Precious opal shows a variable interplay of internal colors, and though it is a mineraloid, it has an internal structure. At the microscopic scale, precious opal is composed of silica spheres some 150 to 300 nm in diameter in a hexagonal or cubic close-packed lattice. These ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.

Historical Significance

The name opal is thought to be derived from the Sanskrit word upala (उपल), meaning 'jewel', and later the Greek opállios (ὀπάλλιος), meaning 'to see a change in color'. In the Middle Ages, opal was considered a stone that could provide great luck because it was believed to possess all the virtues of each gemstone whose color was represented in the color spectrum of the opal.

Types of Precious Opal

**Black Opal:** The most valuable type, with a dark body tone that makes the play-of-color pop vibrantly. **Boulder Opal:** Forms within ironstone boulders; the natural dark backing enhances the color. **White/Light Opal:** The most common type, with a milky white or light gray body tone. **Crystal Opal:** Transparent or semi-transparent, allowing light to pass through.