Inclusions, Cracks and Weight
A well cut stone
adds to the value of the opal. The stone should be well proportioned. A high
dome is nice but not necessary. Excessively fat stones should be faulted if
they make the opal less attractive. A poorly polished surface reduces the ability
of light to enter and leave the opal, thus reducing brightness and value. Inclusions
which are visible on the top of the stone and which detract from the beauty
of the opal reduce value, but inclusions on the back usually do not affect value.
Cracked or crazes
stones are seriously flawed. Opinions vary about their market value, but most
opal dealers consider such stones valuable only as specimens. They are not considered
to be of a quality to use in jewelry and thus have a very low value relative
to a solid stone with the same characteristics.
This is really simple.
Weight is in carats. Small stones of less than one carat are less valuable per
carat and large stones of more than 40 carats are also lower in value.
Boulder opals sell
by the piece instead of by weight. The value of a boulder opal is determined
by the size and quality of its opal face, not its weight. In boulder opals,
size is judged by the area of the opal face on the stone. They are classed as
small, medium, large and very large.
There are a few dealers
who sell boulder opals by weight. If you need to estimate the weight of a boulder
opal, you will have a difficult task. Ironstone is heavy. More importantly,
the weight of a stone of a given size varies significantly with the density
of the ironstone and the amount of it relative to the opal. This is why boulder
opals are usually sold by the piece.