Gemstone Weight Calculator

How the Gemstone Weight Formula Works

The calculator multiplies the stone's volume — estimated as Length × Width × Depth in millimetres — by its specific gravity and a shape factor that corrects for how much of that rectangular volume the cut actually fills. This is the same method gemologists and appraisers use when a stone is mounted in jewellery and cannot be weighed directly on a scale.

Measure with a digital caliper: length across the girdle at the longest point, width at the widest perpendicular point, and depth from the table facet down to the culet.

Specific Gravity of Common Gemstones

Gemstone	Specific Gravity	Gemstone	Specific Gravity
Ruby / Sapphire (Corundum)	4.00	Diamond	3.52
Emerald (Beryl)	2.72	Garnet (Almandine)	3.99
Spinel	3.61	Topaz	3.53
Peridot	3.32	Quartz	2.65

Shape Factors Used

Cut	Factor	Cut	Factor
Round brilliant	0.0018	Emerald cut (step)	0.0025
Oval	0.0020	Marquise	0.0017
Pear	0.0018	Cushion	0.00185

Factors follow the commonly published colored-stone estimation tables (round, oval, pear, marquise and emerald cut). Oval, pear and marquise stones with wide wings or high shoulders can weigh up to 10% more than the estimate.

How Accurate Is the Estimate?

For well-proportioned, calibrated cuts the formula typically lands within 5–10% of the true weight. Results can be further off for very deep or shallow stones, windowed gems, or native cuts. Always confirm final transactions with a calibrated scale.

Frequently Asked Questions

Can I use this calculator for rough gemstones?

The formula is designed for cut and polished stones. Rough crystals have irregular geometry, so treat any result as a rough guide only — and remember that 40–70% of rough weight is typically lost during cutting.

Why does cut shape change the result?

A round brilliant "fills" its bounding box differently than an emerald cut. The shape factor (about 0.0017–0.0025) captures that difference so a long, shallow stone isn't over-estimated relative to a compact one.

What if my gemstone isn't in the list?

Choose "Custom SG…" and enter the specific gravity of your material. SG values for most gem species are published in gemological reference tables.