Standard Gemstone Formula 

Because do not have sufficient data on stones with some cuts, our weight estimates can only use the Standard Formula. Accuracy is not nearly what we would like, and results in larger uncertainty figures being indicated than on other cuts. Naturally, we take no responsibility for your use or misuse of this data. These formulas are courtesy of Robert J. Williams, Ph.D.,G.G. ... Rockhound, Jewelry, Appraising, Retired appraiser of Antiques, Gemstones, and personal property, Author of program Gemstone, which covered over 650 minerals, and all gemstones... a compilation of formulas, data, and instructions for gemologist, mineralogist, and jewelers. Much of the basic data used in the Dendritics Automated Calculations page is courtesy of Bob Williams. (At Debra Willis' suggestion I changed the formulka for oval stones from diameter squared to length * width, which should be more accurate  Thanks, Debra!  JB 22805)
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 Automated Calculations page. 
Images and webpage designs © 20012016 jb and Dendritics Inc. []