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Opals With Curved Backs

I do not understand why opals are cut with a rounded back in the
first place.  Is it simply to give them more weight to charge more
for them?  Would it be advisable to flatten the backs if one has
finished stones that shape? Thank you, 

Since I cut a lot of opals I can give you several reasons (although
most of my opals have flat backs). 1) Curved surfaces are easier to
polish than flat ones. Opal cutters traditionally polished the backs
to provide a more finished appearance because opals were set in
open-back settings. This was not a problem as long as the edges of
the stone fit into the setting accurately so it didn’t break due to
uneven pressure during prong-pushing. Bezel-setting, while
occasionally done, was not commonly seen with opals until a few years
ago. Most of the opals I see offered for sale, especially calibrated
sizes, are cut with flat backs these days. 2) As with star and
cat’s-eye stones, sometimes additional depth is required to intensify
the optical effect – although I’ll admit some Asian cutters really
overdo what I call the “Buddha Belly” on phenomenal stones like star
sapphires, opals, etc. (I’ve seen star sapphires with four times as
much volume below the girdle as above!) But trying to generalize
about opals is hopeless because there are many varieties, and tricks
to cutting each one. Ask your opal supplier for an explanation of
why stones are cut in a particular fashion. There are usually very
good reasons. If you don’t think the explanation holds up, make an
offer on the stone based on its weight after recutting.