All opal formed naturally is known
as "natural" or "solid" opal.
This is still the case, even when the stone is not opal all the
way through, but has a natural backing of potch or ironstone.Most
solid black opals and boulder black opals are cut with some of their
original potch or ironstone left in place on the back to give the
opal more strength for setting and provide a dark background for
the colours of the opal.
Since the 1960's, some synthetic and manufactured forms known as
Gilson doublets and triplets have also been developed to have an
"opal-like" appearance. Doublets and triplets are not
solid opal all the way through and are manufactured by laminating
a thin slice of opal onto a black plastic or potch base and then
adding a clear quartz dome on the top in case of triplets.
These man-made forms can be extremely sensitive to water, humidity
and heat, which may cause the different layers to seperate. However
they can be very beautiful as costume jewellery and are often sought
after as souvenirs.
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