Opal is a hydrated amorphous form of silica. It is classed as a mineraloid rather than a mineral, because of its amorphous form, unlike crystalline forms of silica, which are classed as minerals.Opal is known for its unique display of flashing rainbow colors called play-of-color, known as “opalescence”. There are two broad classes of opal: precious and common. Play-of-color occurs only in precious opalas it consists of sub-microscopic spheres stacked in a grid-like pattern causing interference, refraction and diffraction of light. Other varieties of opal are Fire opal,Girasol opal and Peruvian opal.

Opal Deposits

Australian opal has often been cited as accounting for 95–97% of the world's supply of precious opal.Some of the famous Australians mines are Coober Pedy, Mintabie,Jundah, Andamooka, Lightning Ridge, Koroit, and Quilpie.Other deposits are found in Brazil, Ethiopia, Guatemala, Honduras, Indonesia, Japan,Mexico, Russia

Opal Hue

Opal can occur in many colors including white, colorless, pale yellow, pale red, gray, brown and black. The black opals are the rarest, whereas white and greens are the most common. The unique Play-of-colors is shown by precious opals.Opalescence is usually milky blue in color, but it can occur in rarer colors.

Opal Clarity and Surface

Opals vary in optical density from opaque to semitransparent.Translucent materials are typically more valuable than opaque materials. Almost all opal shows some visible internal fractures or inclusions. Opals exhibit a waxy to resinous luster which helps accentuate play of color and opalescence. Finer stones often have a vitreous luster.

Opal care and cleaning

• Opals have water content between 6 and 10%. When allowed to dry, it will crack and fade.
• If you live in a very dryclimate, keep them in a tight plastic bag, with a damp piece of cotton or fabric.This will prevent dehydration.
• At 5.5 to 6 in hardness,opal can scratch easily. Even wiping off the dustwill gradually reduce its polish as ordinary dust, quartz has mohs hardness of 7.
• Clean your opals using a soft cloth and only mild soap when needed.

Opal gemological properties

Chemical Formula: SiO2_nH2O - Hydrous silicon dioxide

Crystal Structure: Amorphous; kidney or grape-shaped aggregates

Color: All colors, partial play of color

Hardness: 5.5 to 6.5 on the Mohs scale

Refractive Index: 1.37 to 1.52

Density: 1.98 to 2.50

Cleavage: None

Transparency: Transparent to opaque

Double Refraction: None

Luster: Waxy to resinous

Fluorescence: White opal: white, bluish, brownish, greenish; fire opal:greenish