The opal pricing is based on 6 main criteria; clarity, carat, colour pattern, colour, opal type, and brightness.
CLARITY: the purity of opal is measured by the number and prominence of inclusions consisting of sand or colourless opal. (generally, the fewer inclusions the higher the price)
CARAT: weight of the stone (generally the price increases as the weight increases).
COLOUR PATTERN: opal colour patterns vary drastically in rarity and appearance. (rarer patterns result in higher pricing)
COLOUR: colours play a significant role in the process of determining the value of opal, as a rule of thumb red is one of the rarest and most expensive colours however unique combinations of colour will also contribute to the value.
OPAL TYPE: the main opal types include: white/ light opal, crystal opal, black opal, and boulder opal. The origin of the opal plays a role in the type of opal for example Lighting Ridge, NSW is known for black opal, Coober Pedy, SA is known for crystal opal and Outback QLD is known for boulder opal. (generally, the dark based opals are worth more than the lighter based opals however the categories mentioned previously impact the pricing significantly)
BRIGHTNESS: the brightness refers to the intensity of the opal's colour on a scale consisting of SUBDUED - MODERATELY BRIGHT - BRIGHT - VERY BRIGHT -VIVID. Subdued being the least and vivid the most bright.