Online Gem Weight Calculator

Notes:
Volume Factor:      Width:      Material:
    

Using the Gem Weight Calculator:

  1. Enter optional notes, such as "Gem weight calculation for 16mm Sparkler".
  2. Enter the design volume factor (Vol./W^3).
  3. Enter the width of the stone.
  4. Select measurement units (millimeters or inches).
  5. Select the material.
  6. Click the 'Submit for Processing' button at the bottom of the form when you've finished entering the above information.
  7. If you want hardcopy of the computed weight, just print out the results page using your browser's print function.

The weight of a stone cut from a particular material can be calculated as the product of the stone's volume times the specific gravity of the material. The volume expressed as a percentage of the width cubed is constant for a particular design, and is given in GemCad rendered diagrams as the volume factor: Vol./W^3. Given the volume factor, the width of the stone in millimeters, and the material (specific gravity), the stone's weight in carats can be calculated as:

Weight = (Vol./W^3) x (specific gravity) x (width3) / 200

If your stone is cut per the planview and angles as given in the GemCad diagram, and your measurement of its width is accurate, the calculator and a scale should agree pretty closely.

The volume factor calculated by GemCad is the percentage of volume the stone would occupy within a cube of material with edges equal in length to the stone's width. The volume factor of your cut stone and the virtual stone only deviate to the extent that the angles you actually cut deviate from the GemCad diagram angles. This deviation should be pretty small if you're cutting per the diagram. If you modify or tangent ratio the design, that affects the volume factor. If you make the changes to the virtual stone using GemCad, it of course computes a new volume factor.

Specific gravity is the ratio of the weight of a volume of a particular material to the weight of an identical volume of water. If you know the specific gravity of your rough, the GemCad volume factor for the design can be used to calculate a finished weight for a stone of any particular width. To make this online weight calculator as easy as possible to use, it provides a drop down list of materials to select from instead of requiring you to know and input the specific gravity of your material.

Some gem materials (minerals) have unique compositions and their specific gravities are also unique. Others are in solid solution series with other minerals or members of chemically related families and their composition is variable, as are their specific gravities. Garnet and tourmaline are two examples of such gem materials.

This calculator primarily uses specific gravity values and ranges for the various materials as given in the Photo Atlas of Minerals for the natural materials listed in the pull down menu, and Vargas for the synthetics. Mineralogy and faceting references sometimes don't agree as to specific gravities and it's possible the specific gravity of your material could vary from the look up value or range used by the calculator. In calculating the volume factor of a design GemCad assumes a girdle thickness equal to two percent of the stone's width, so the actual girdle thickness of your own stone is an additional variable affecting the accuracy of the calculated weight.

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Bob Keller