Gemologist's Toolkit

Quick-reference calculators and lookup tables for gemstone identification

Hydrostatic weighing

Weight in air (g)

Weight submerged in water (g)

Formula: SG = W_air / (W_air − W_water)
Use a scale precise to at least 0.001 g. Submerge the stone fully in distilled water at room temperature, touching nothing but the suspension wire or basket.

Reference: common gem SG values

Gem	SG range

Refractometer reading

RI reading (or low reading if birefringent)

High reading (optional, for doubly refractive stones)

Standard refractometers (with methylene iodide contact liquid) read up to about RI 1.81. Stones above that — diamond, zircon, cubic zirconia, moissanite — read "over the limit" (OTL) and need other ID methods.

Reference: common gem RI values

Gem	RI range

Estimate weight from dimensions

Cut / shape

Diameter (mm)

Width (mm)

Depth (mm)

Material

Formula: Weight (ct) = L × W × Depth × shape factor × (SG ÷ 3.52)
Shape factors are the standard diamond-calibrated constants (round uses average diameter squared instead of L×W); results are an estimate, not a substitute for weighing an unmounted stone.

Mohs hardness reference

Gem	Hardness

Higher numbers scratch lower ones. Everyday reference points: fingernail ≈2.5, glass ≈5.5, steel file ≈6.5.

Weight unit converter

Carats (ct)

Points (1 ct = 100 pt)

Grams (g)

Milligrams (mg)

Pearl grains (1 gr = 0.25 ct)

Edit any field — the rest update automatically.

Birefringence & pleochroism reference

Gem	Birefringence	Pleochroism

Isotropic stones (diamond, spinel, garnet, cubic zirconia) show no birefringence or pleochroism. Values are typical ranges — individual stones vary.