Opal 

Opal is the modern October birthstone and the accepted gem for the 13th wedding anniversary.

Most Opal is 50-65 million years old, dating back to the Cretaceous period when dinosaurs roamed the earth. 

Opal formed as silica from decomposing rocks mixed with ground water which formed a silica gel that collected and hardened in underground cavities and fissures.

There are two distinct types of Opal, common and precious. The way the silica particles form determines which type.  In precious Opal, silica particles are packed in regular rows and layers.  Moving the stone causes light to diffract, or split, as it grazes the Opal surface.  This light diffusion shows iridescent flashes of green, blue, aqua and sometimes yellowish or red colours which are referred to as "fire".

Opal's hardness on the Mohs scale is 6.5, however Opal is very fragile and can easily be cracked or broken on impact.  Care must be taken in its wear.

Colour and Varieties
 
Black Opal  comes from the Lightning Ridge region of Northern New South Wales.  It is the most rare and most valuable Opal.  Fine quality black Opals often cost more per carat than diamonds.  The term black Opal is misleading because the Opal is not actually black, but instead has a very dark base. This dark base enhances the brilliant colours know as "fire".

Boulder Opal  occurs in the boulder Opal belt, an area between the New South Wales border and northwest Queensland.  Boulder Opal forms on a dark ironstone base (the host rock) and occurs as a thin uneven layer adhering to the ironstone.  Because of the uneven layers, sometimes part of the ironstone is visible on the surface of the stone.  Queensland Boulder Opal accounts for a small percent of the world market, and because of its brilliance and rarity can bring up to $1000 a carat.

Light Opal  is the most common type of Opal found and refers to both the crystal type Opal (translucent) and the milky Opal (opaque).  It is the most common variety and is generally less valuable than gem quality light Opal.  Brilliantly coloured light Opals may be quite expensive with a value exceeding some Black and Boulder Opals.

Fire Opal
Brilliance or lustre is one of the primary factors that determines an Opal's value.  An Opal with strong intensity and colour play adds more value.

Legend and Healing Properties

Opal takes its name from the Latin word Upala meaning precious stone.  The Roman scholar Pliny described Opal as having the fire of carbuncle (a deep red garnet) and the brilliance of amethyst with the green colour of emerald. 

The Romans believed Opal was the symbol of hope and purity.  They called it cupid paederos (child as beautiful as love) and thought the wearer safe from disease.  The Arabs believed that Opals fell from heaven in flashes of lightning.  Ancient Greeks also believed the Opal had the power of giving foresight and the light of prophecy to their owners.  Cleopatra is said to have worn an Opal to attract the attention of Mark Anthony.

Superstition bad luck?

There is a superstition that suggests it is bad luck to wear an Opal, if Opal is not your birthstone.  This superstition is not rooted deep in history but only goes back to the early or middle 19th Century.  Sir Walter Scott's novel Anne of Geierstein, published in 1831 is the source.  It seems that Anne had an Opal that that reflected her moods: it shone red when she was angry, blue when she was sad and green when she was happy.  When Anne died, the Opal faded and lost all of its colour.