Opal can be common or precious. Common opal, called "potch" by miners, does not show the display of colour exhibited in precious opal, or the "play of colour" which is later described, however some specimens of common opal are attractive and colourful. It is given the name common as it can be found in many locations across the world.
Precious opal is mined in Australia – which produces 97% of the world's supply and is the country's national gemstone. The internal structure of precious opal makes it diffract light; depending on the conditions in which it formed it can take on many colours ranging from clear through white, grey, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive brown and black.
Of these hues, red and black are the rarest, white and green the most common. Precious opal flashes iridescent colours when it is viewed from different angles, when the stone is moved or when the light source is moved. This phenomenon is known as a "play of colour", which makes opal a popular gem.
There are many different types of opal and their individual attributes are listed below:
WHITE or LIGHT: Translucent to semi-translucent, with play-of-colour against a white or light gray background colour, called bodycolour.
BLACK: Translucent to opaque, with play-of-colour against a black or other dark background.
FIRE: Transparent to translucent, with brown, yellow, orange, or red bodycolour. This material—which often doesn't show play-of-colour—is also known as "Mexican opal."
BOUDLER: Translucent to opaque, with play-of-colour against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
CRYSTAL or WATER: Transparent to semi-transparent, with a clear background. This type shows exceptional play-of-colour.
The first references to opal in the Roman ages are around 250 BC, at a time when opal was valued above all other gems. The opals were supplied by traders from the Bosporus; a strait (also known as the Istanbul Strait) that forms part of the boundary between Europe and Asia, who claimed the gems were being supplied from India.
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 colour was represented in the colour spectrum of the opal.
It was also said to confer the power of invisibility if wrapped in a fresh bay leaf and held in the hand. However, following the publication of Sir Walter Scott's Anne of Geierstein in 1829, opal acquired a less auspicious reputation. The Baroness of Arnheim in the novel dies soon after the opal she wears comes into contact with holy water and due to the popularity of Scott's novel, people began to associate opals with bad luck and death which saw sales of opal in Europe drop by 50% and remained low for the next 20 years or so.
The Olympic Autralis Opal found in 1956 in the Eight Mile in South Australia is the largest and most valuable opal yet found, valued at AUD 2,500,000. Found at a depth of 9.144 metres, it was named in honour of the Olympic Games which were being held in Melbourne at the time. It consists of 99% gem opal with an even colour throughout the stone, and has been left in a natural organic state, unpolished and uncut with blemishes. Its dimensions are 280mm in length x 120mm in height x 115m in width, weighing 17,000 carats (3450 grams).