COMMON OPALS and POTCH
Do not show a play-of-colour.
opals may be valued for a pleasant body colour e.g. pink, blue and green
(Coloured Opals) or red, orange and yellow (Fire Opals). Fire Opals
frequently show also a play-of-colour in which case they are considered
Common Opals may also be of interest in the form of opalized fossils (e.g.
shells, snails, belemnites, bones, teeth and wood) and as pseudomorphs
of opal after glauberite crystal aggregates (Opal Pineapples). Such
fossils may also exhibit a play-of-colour and be classed as Precious
Opals. Furthermore Common Opals may be praised for dark dendritic inclusions which
resemble the patterns of mosses, ferns or trees (Dendritic Opals) and for an
optical cat's eye effect (Cat's eye Opals) that is due to many fine,
parallel arranged needle or tube like inclusions (this effect must not
be confused with a rolling flash). Also black potch may be used as
the backing of
doublets and triplets.