Gemstone Carat Weight Calculation from Dimensions

Question
I am trying to figure out how to determine the carat weight of a gem stone when the advertised size is in mm. ie( 7mmx9mm)?  I realize that without depth etc. of the stone it would be an approximation, but wonder if you can help?  Thanks

Answer
Hi Kate, A little complicated to a non trades person but it depends on two major variables. The shape and the specific gravity of the particular gem. (density within mass)  Rounds, ovals, square cushions, rectangular cusions, and square emerald cuts the formula is diameter x diameter x depth x SG (specific gravity-.0018 to .0024). Fancy shapes ie: pears, hearts, marquise, triangles, and cabochons the formula is length x width x depth x sg x .0018-.0029. A leveridge guage is used in the trade to do this and get a close estimate but weighing the stone loose is the best alternative. A jeweler can save you a lot of time. A traditional 9x7 is about 1.5 cts.

Steve Kretser