A note about stone sizes and weights
Emeralds, Rubies, and Sapphires are not standardized as to how they are cut,
so two different stones of identical quality-- say, two rubies
-- although both of them may be cut exactly 3 millimeters across,
both of them may be cut to different depths. Thus one will weigh
more than the other and will cost more. When you are buying emeralds,
rubies, and sapphires, you have to expect some variation in weight
and price even though the stones measure the same size across.
In addition, the quality difference between two stones exactly
the same size can greatly affect cost.
Another factor in understanding gemstone sizes and weights is
the fact that the minerals that make up different gemstones may
weigh different amounts. A comparison may be made between rubies
(the red variety of the mineral "corundum") and emeralds
(the green variety of the mineral "beryl"). Corundum
weighs more than beryl. So a ruby cut exactly 3 mm across and
exactly 3.25 mm deep will weigh more than an emerald cut exactly
the same size. And an emerald cut to exactly the same size as
a diamond will weigh slightly more than the diamond. You will
notice these seeming discrepancies in the stone size charts.
Diamonds are much more standardized as
to how they are cut, because they can lose much of their ability
to sparkle if cut too shallow or too deep. Therefore, one diamond
of decent quality that measures 3 mm across will likely be cut
to about the same depth as another 3 mm diamond of the same quality
and will likely weigh very close to the same.