Many factors must be considered when valuing opal.  Black or dark opal is in general more valuable than light opal. The intensity or brilliance of colour is of most importance.  The finest of opals are maximum brightness and clarity. There should be an even play of colour all over the stone with no patches of colourless opal.  For the highest value, all colours should be present, especially red which is the rarest.  The stones should be free from obvious flaws on the face of the opal.  Marks on the base usually have no effect on price.