Body tone – The darker the body tone, (backing of the opal) the more valuable the opal is .
Directionality – When you turn the opal. Is there any “dead “ spots. If there is no “dead” spots the opal is more valuable.
Depth of colour - The deeper and brighter the colour is the more valuable the opal is.
Colour – The blue colour is the most common, then green , yellow, orange and red. A opal with all colours is more valuable than a single colour opal.
Harlequin — A harlequin pattern is made up of a patchwork of irregular-sized squares of colour.
Pinfire — A pinfire pattern consists of closely spaced pinpoints or specks of colour.
Flash — A flash pattern shows broad irregular flashes of colour which change or disappear as the stone is rotated.
Chinese writing – looks like Chinese writing across the face of the opal.That is just some of the patterns, the rarer the pattern is the more value the opal has. A pinfire pattern is a lot more common than Chinese writing or Harlequin, so therefore not so valuable. This is just a few examples of patterns.there is many more.
Windows - Many opals, especially those with a grey or dark background, have paler areas showing through the colour. These are usually the result of the potch being unevenly coloured at the back of the stone, or of sand intruding from the back into the colour layer. These will reduce the opal's appeal, and hence its value.
Weight –finally the size of the opal. It is Weighed in carats. 1 carat is 0.2 grams. Boulder opal is an exception because it has the iron stone as the host stone.