Gem Stones Carat

Carat is a measure of weight not size as is commonly thought.
A carat = 200mg or 1/5g. 1ct = 100pts. If a gem were to weight 100mg = 50pts = 1/2ct.

As different gems have different densities, a half carat diamond will have a different size to a half carat sapphire. It is also not always the case that two diamonds with the same diameter will weigh the same. As a gem is three dimensional, a deep gem weighing 0.5ct will have a small diameter, a shallow gem weighing 0.5ct will have a large diameter. There is of course an ideal cut which maximises the gem's beauty.