Advanced Gemstone Formula

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Standard Formula

The Standard Formula for calculating the mass (volume actually) of gemstone cuts has been known for some time, and produces reasonable results in many cases. 

Advanced Formula

More sophisticated estimates are available for certain cuts, in which the effects of belly shape, culet size, girdle size, etc., can be adjusted for.  We call this our "Advanced Formula", and the results are available in our Carat Weight Estimation page.  The parameters were carefully optimized using data on over 1,000 individual characterized gemstones.   By performing multi-dimensional curve fits to the data, it was possible to separate out the effects on volume of a whole variety of different shape factors. 


For example, cutting a larger culet might not seem to have any significant effect on the mass of a stone, but if you think about it, what it means is that the depth measurement, which is taken from the culet to the table, will be different from what you would otherwise get on the same stone with a smaller culet.  The Depth will read smaller.  Using the Standard Formula's basic Length*Width*Depth metric, this will result in a significantly lighter estimate of the stone.  Our formula corrects for this effect, so the culet size entry in effect modifies the Depth measurement you have specified.  Similar changes happen for the other shape parameters.


In addition, testing this formula on such a large set of characterized stones has given us the data to be able to grade our own results, so you will see not only a Best Estimate of the weight, but a tolerance on this number, which takes into account the known error level we experienced on the stones used in compiling the data, as well as the tolerances reported to us from the field in specific gravity (or density) figures for the gemstone species you have selected.  Thus, you will find better results for Diamonds than for Quartz, as Quartz has such a wide range of specific gravity.  This improvement will show up in the tolerance reading we report.

Accuracy / User Beware!

This Formula has proven accurate in our tests. It is possible, however, that there is a misteak in our data or estimation algorithm or that your stone may lie outside the size range of our tests.  Always use common sense in evaluating the results of computer calculations.  Computers are amazingly fast, very obedient, and incredibly useful tools.  But they are dumb beyond comprehension.  Our server will happily estimate a weight for your stone, but it only knows what it has been told.  It has never seen a gemstone, and has no common sense about what is reasonable or absurd.  Please inform us of problems.   Naturally, we take no responsibility for your use or misuse of this data.

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