Opal

Introduction

October birthstone. From the Sanskrit 'Upala' meaning precious stone. Not a crystal but formed as a microscopic array of minute spheres of silica, of uniform size, which reflects & breaks up light to give the characteristic play-of-colour. When it shows this iridescent effect it is called precious opal. Different colours are related to the different sizes of spheres.

Value is based on type, background colour, brightness of play-of-colour. Red is deemed the rarest of the iridescent colours but those with strong, vivid colours against dark background are judges more valuable.

Properties: Mohs 6 - soft; have water in their composition so can dry up and crack (=crazing), should be stored away from heat.

Lore: Some said to be unlucky - Sir Walter Scott's 'Anne of Geierstein'. the character Hermione had an opal that changed with moods. When her daughter was christened the ring was touched by water and lost colour. Later Herm. was reduced to ashes. People stopped buying opals after this was published. In early 19th cent. thought to be lucky as it had the power and colours of the gemstones.

Is commonly treated (with wax or plastic to prevent & disguise cracking) and imitated. Treatments can aim to darken the background of paler stones to show the colour effect better. Can produce composite gemstone - doublet (stone & dark backing) or triplet (inc. a transparent upper layer).

White opals forst mined in Czechoslovakia in the 14th cent. but Australian deposits brought opal to the market. Aust. has produced the most magnificent eg: harlequin opal (rare). Also found USA, Brazil & Honduras, Welo in Ethiopia more recently (1990s).

Clean in tepid water with soft cloth, but do not clean a composite stone in water. Don't leave in the sun.

Types of Opals

Opals come in various types, named for body colour, each with unique characteristics. The main categories include:

Precious Opal (displays a play-of-color, which is the iridescent effect seen in some opals). Types:

• Black Opal: Has a dark body tone, enhancing the visibility of colors. Considered the rarest and most valuable.
• White Opal: Features a light background with play-of-color. Commonly found and less expensive. Mostly mined in Cooper Pedy, SA; Wollo Province, Ethiopia; Spencer, Idaho. Play of colour is more supdued than that of black opal.
• Crystal Opal: Transparent to semi-transparent, showcasing vibrant colors against a clear background.
• Boulder/Matrix Opal: Contains opal within a host rock, often retaining some of the original matrix. The opal is cut with the matrix.
• Water Opal: Transparent or colourless with few or no inclusions.

Common Opal (lacks the play-of-color seen in precious opals) Types:

• Milk Opal: Opaque with a milky appearance, can sometimes be of gemstone quality.
• Fire Opal: Brown, yellow, orange or red. May show faint play-of-color. Transparent to translucent.
• Wood Opal: Formed from the replacement of wood with opal, often displaying unique patterns.

Synthetic Opal (man-made opals created to mimic natural opals): Often have a regular pattern and lack inclusions found in natural stones.

Other Varieties:

• Hyalite: Colorless and glass-clear, sometimes called Muller's glass.
• Geyserite: Deposited around hot springs or geysers, often with a unique texture.
• Dendritic: or moss opals show black, fern-like manganese or brown moss-like inclusions. From many locations.