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
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
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Key Differentiators
- Hardness 3 — a copper coin scratches it
- Perfect rhombohedral cleavage in three directions
- Extreme doubling: a clear rhomb doubles text placed beneath it
- Effervesces weakly in dilute hydrochloric acid when powdered
Treatments
- Dyeing (massive material, to imitate jadeite)
- Wax or Plastic Impregnation
Measurement Guides
Identifying a calcite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →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.
Practice exams →