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, (who was pretty darn old in 1996 and is most likely dead now, RIP). 

Round faceted stones:

Diameter squared x depth x S.G. x .0018 = carat wt.

Oval faceted stones: (repaired 02-28-2005, Formula changed from diameter squared to length * width, Thanks, Debra Willis!)

Length x width x depth x S.G. x .0020 = carat wt.

Emerald cut faceted stones:

Length x width x depth x S.G. x .0025 = carat wt.

Rectangular faceted stones:

Length x width x depth x S.G. x .0026 = carat wt.

Square cut faceted stones:

Length x width x depth x S.G. x .0023 = carat wt.

Marquise cut stones, (navette):

Length x width x depth x S.G. x .0016 = carat wt.

Pear cut stones:

Length x width x depth x S.G. x .0018 = carat wt.

Square cushions: (average corner meas.& belly meas. for diameter)

Diameter squared x depth x S.G. x .0018 = carat wt.

Rectangular Cushions: (average  L & W for diameter)

Diameter squared x depth x S.G. x .0022 = carat wt.

Square step cut stones: (average  both meas. for diameter)

Diameter squared x depth x S.G. x .0023 = carat wt.

Rectangular step cut stones:

Length x width x depth x S.G. x .0025 = carat wt.

Cabochon cut stones:

Length x width x depth x S.G. x .0027 = carat wt.

Spheroids: (repaired 10-28-2023)

Length x width x depth x S.G. x .0021 = carat wt.

Rectangular solids: (repaired 10-28-2023)

Length x width x depth x S.G. x .0050 = carat wt.

Octahedrons: (repaired 10-28-2023, Thanks, David Atlas, for discovering my gross oops!)

Length x width x depth x S.G. x .0043 = carat wt.