The type, colour, size and soundness of precious opal are factors that determine the price paid for the gemstone. The price is based on the quality of the opal and expressed per carat. Furthermore, there is a marked difference between the value of uncut opal compared with the value of cut and polished opal. Categories such as the variety, background, transparency, spectrum, tone, origin, distribution, inclusions, carat weight etc all add to determining the correct value of the opal itself. The clarity of the colour is critical when assessing the value of opal. Red fire is the rarest colour, followed by green/orange, green/blue and blue. Therefore red fire opal is generally more valuable than a predominantly green opal, which in turn is more valuable than a stone showing only blue colour.