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Calcite

Last updated: September 2026

Calcite is the classic demonstration of double refraction. A clear rhomb — the variety called Iceland spar, first worked from the Helgustadir mine in Iceland — doubles any text placed beneath it, and Nicol cemented two pieces together to build the first practical polariser. Its birefringence of 0.172 is among the largest of any gem material that a standard refractometer can still show in full, because both indices fall below the 1.81 contact-liquid ceiling. Sources give dispersion separately for each ray, 0.008 for the extraordinary and 0.017 for the ordinary; the ordinary figure is the one recorded here. Perfect cleavage in three directions and a hardness of only 3 make it one of the hardest of all minerals to cut, so faceted stones are collector pieces rather than jewellery.

Calcite has a specific gravity of 2.71 and a refractive index of 1.486–1.658, with a Mohs hardness of 3. It is uniaxial −.

Physical & Optical Properties

RI Range1.486–1.658
SG Range2.67–2.73
SG Typical2.71
Hardness (Mohs)3
Crystal SystemTrigonal
Optic CharacterDR Uniaxial (−)
Birefringence0.172
Dispersion0.017
Fluorescence LWVariable
Fluorescence SWVariable
Chelsea FilterVariable
PleochroismWeak
ColorsColorless, Yellow Orange, Green, Red Pink, Blue Violet, Brown, Multi
SpeciesCalcite
ColorlessYellow OrangeGreenRed PinkBlue VioletBrownMulti

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe and a darkfield loupe — step-by-step for each.

Through the refractometer

Calcite reads 1.486 – 1.658 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, reading both edges atevery 45°. SEETwo boundaries, 0.172 apart at their widest separation. MEANSThat gap IS the birefringence. Take max minus min across thewhole rotation — a single position understates it, so onereading is an incomplete one. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

Through the polariscope

Calcite Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Calcite blinks light-dark four times in a full turn. That is doubly refractive. It does NOT tell you uniaxial from biaxial — that is the optic figure, which needs a conoscope. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Calcite SG 2.67–2.73 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 19 other species in the table overlap this range, so a reading here narrows the field to 20 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Calcite weak — colours not recorded COLOURS NOT IN OUR SOURCE Calcite is recorded as showing weak pleochroism, but our source does not name the two colours, so this figure does not paint them. The windows will differ; which hues they take is not something we hold. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

Key Differentiators

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a calcite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Calcite is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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