Opals were precious stones in the Tortallan Universe. They were very valuable, not only because they were pretty and rare, but also because they had various uses in magic. Opals were often associated with the Trickster, because they were unpredictable. Some lasted for years or even decades, while others broke within only a few months. Opals had to be treated especially careful after being mined and it was best to store them for at least a year before cutting them. They were especially prone to cracking when they were in long contact with water and left to dry.
There existed differend kinds of opals, mined in different parts of the Tortallan Universe. In Tortall, around and in its capital Corus, fire opals were found in the early half of the third century of the Human Era. At that time black opals were mined in northern Galla, two hundred years later those opals could also be found in the Tortallan fief Dunlath. Carthak was also known to turn out opals, among them sirajit opals, which could mainly be found in two different colours: tawny--the colour of those stones resembled those of a Yamani's skin--and dark--those opals were of a dark mahogany brown. Clear sirajit opals, nearly white with colourful sparks in them, are more rarely. Dale Rowan once presented Rebakah Cooper with a silver bracelet with sirajit opals, one of them was a clear gem. A trait which made the opals especially pretty were the different coloured fires within them.