The easiest way is to buy a carat scale. The electronic ones will
usually convert between grams & carats at the push of a button.
One other way is to use a gram scale capable of measuring milligrams.
A carat equals 1/5 of a gram. If a gram scale is used to measure
carats, it should be capable of measuring to 1/100 (1 point) of a
carat. 1 point is 1/500 of a gram. Some gemstones get very expensive &
being off, weight wise, can cost you a lot of money when buy or
selling a stone.