Gemologist's Toolkit

Quick-reference calculators and lookup tables for gemstone identification

Enter known values

Fill in whatever you've measured — matches narrow as you add more. Suggested order: optic character (polariscope), refractive index, birefringence, specific gravity, hardness, pleochroism, heat signs.

Optic character (polariscope)

Refractive index (low reading)

RI high reading (optional)

Birefringence (DR)

Weight in air (g)

Weight in water (g)

Specific gravity

Mohs hardness

Pleochroism observed

Signs of heat treatment

Possible matches

Gem	Optic	SG	RI	Hardness	Pleochroism

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.

Optical properties reference

Gem	Optic character	Birefringence	Pleochroism

Isotropic gemstones (e.g. diamond, spinel, garnet, cubic zirconia) have the same refractive index in every direction and therefore have no optic axis — normally singly refractive and non-pleochroic, though internal strain can cause anomalous reactions in a polariscope. Anisotropic gemstones have one or two optic axes and are normally doubly refractive, except when viewed exactly along an optic axis; coloured specimens may show pleochroism. Values below are typical ranges — individual stones vary.

Treatments & signs of heating

Gem	Commonly heated?	What to look for

These are trade-common tendencies, not guarantees — any individual stone can be untreated or treated against type. Absence of visible indicators doesn't prove a stone is untreated; confirming treatment status reliably usually needs lab testing.