A fire opal is a gemstone which is very valuable in Tortall not only because they are beautiful but also because they are very difficult to cut. Normally "only a trickle of new gems enters the market in any one year".
Even uncut the stones are already quite pretty and spark bits of coloured fire from their bed. They shimmer in all sorts of different colours. When being made wet and hold before the light they are even more beautiful and colourful.
In 246 HE, Crookshank discovers that the sandstone-foundations of the Lower City of Corus contain raw fire opals. That is why he hires diggers to mine the stones in the cellars of his houses, later having them killed and thus committing the Opal Murders. He also planted the rumour of an auction for fire opals in August of 246 HE. That's why several foreigners, among them Carthakis and Yamanis, visited Corus, even months before the rumored auction.
Fire opals are fascinators, bewitchers; they can be used in magic to take and hold the attention of anyone the mage shows them to, leaving them open for suggestion. The opals have to be properly spelt for this and can only hold the spell for a few days. Fire opals crack when they are left in the water and afterwards being left to dry out. They can't be used for spells anymore.