Standard Gemstone Formula

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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 2-28-05)

Round faceted stones:

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

Oval faceted stones:

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.

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.

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