Opal

According to Arabic legend, opal was believed to contain lightning.

Writers have compared opal’s color to volcanoes, galaxies and fireworks. Admirers gave extraordinary opals poetic names like Pandora, Light of the World and Empress. In ancient Rome, this gem symbolized love and hope. The Romans gave it a name—opalus—that was synonymous with “precious stone.” Many cultures have credited opal with supernatural origins and powers. The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease. Europeans have long considered the gem a
symbol of hope, purity and truth. 

Play-of-Color
Opals display a phenomenon known as play-of-color. When a stone has play-of-color, it is referred to as precious opal. The main categories of precious opal are white, black, boulder, and crystal or water. Fire opal, also known as Mexican opal, sometimes doesn’t show play-of-color.

Clarity
With an opal, clarity is its degree of transparency and freedom from inclusions. An opal’s clarity can range all the way from completely transparent to opaque.
A cloudy or milky background color can sometimes signal a lack of stability. Opals can have fractures and surface blemishes. Matrix, or host rock, along with signs of crazing, a fine network of cracks, have an impact on opal’s durability and value.

Sources
Fine opal comes from all over the world. The most significant sources are Idaho, Australia, Mexico and Ethiopia. 

Treatments
Opals can be treated by impregnation with oil, wax or plastic. Opal doublets or triplets are thin slices of opal glued to a base material and covered with a thin dome of clear quartz. These gems are more resistant to scratching but are considered less valuable. Some opal can be dyed.

Major Sources
Idaho, Australia, Brazil, Ethiopia, Mexico,
Peru, United States 

Mineral
Hydrated Silica

Mohs Hardness
5 to 6.5

Toughness
Very Poor to Fair

Color
All colors

Birthstone
October

Care and Cleaning
The only safe way to clean opal
is with warm, soapy water. Opals
are sensitive to heat or excessive
dryness which can lead to crazing
or internal cracks.

 

Source: The Passion of Colored Gemstones by G.I.A.