Calcite

Calcite is a mineral named after its calcium content. It is one of the most common minerals found in abundance on the Earth. Iceland spar is a colorless variety of calcite, which is well known for its very strong doubling (double refraction).

Due to the low hardness of calcite it is not sought as gem quality mineral as it is prone to scratch but still it is used as cabochons, facetted cuts and carvings in gem industry while it is also used in production of lime which is further used in steel and glass industry. The iridescent effect seen at cleavage cracks of calcite is also popularly known as rainbow effect in trade

Common worldwide, calcite is the principal component of limestones and marbles, and of most stalactites and stalagmites. It can also be found as large, transparent, colourless, complex crystals, or as prismatic crystals intergrown with other minerals. Because to its softness it is only faceted for the collector, but marbles and brown, banded calcite from limestone caves are both used for decoration and carving.

Italy is known for high-quality marbles, particularly the creamy Carrara marble. Transparent, colourless rhombs are classified as "Iceland spar"; a white fibrous variety, when cut in cabochon, shows the cat’s-eye effect. Pink and green crystals occur in the United States, Germany, and England.

Calcite roughs are found in rhombohedron forms but due its perfect three directional rhombohedral cleavage and low hardness the calcite rough is cleaved within the ore or during the mining process, and does not show a complete rhombohedron. But while carefully examining it still can be figured out on a whole as a rhombohedron form.

How is calcite formed?

It is calcite (calc - calk) that has been dissolved by the acidity in rain/water passing through limestone. It then drips from cave roofs and forms calcite straws which often become blocked and then form stalactites as the calcite forms around the outside of the straw. The water dripping from the straw or stalactite deposits calcite on the floor which becomes a stalagmite or flowstone if it runs at an angle. The red in the calcite is due to ironstone in that area.

Calcite Pictures

Calcite crystals Orange calcite Yellow calcite

Iceland spar calcite Druze of calcite and quartz from Arnenia

Calcite Properties

Chemical Composition: 
CaCO3 + Mn, Zn, Fe, Co (may substitute for Ca). Dimorphous with aragonite.
Colors / Varieties: 
  • Color:
    • White, yellow, brown, pink, blue.
  • Varieties:
    • Iceland spar - Transparent (colorless).
    • Onyx marble - Transparent to opaque with irregular banded patterns.
    • Satin Spar - Aggregate made up of fibrous crystals.
Crystal System / Forms: 
Trigonal System / Varied forms, rhombohedron, scalenohedron, contact and repeated twinning.
Hardness: 
3
Specific Gravity: 
2.58 - 2.75 (2.71 mean)
Cleavage / Fracture: 
Perfect 3 directional rhombohedral cleavage / Uneven
Optic Character: 
Anisotropic (D.R.); Uniaxial negative.
Lustre: 
Vitreous.
Refractive Index / Birefringence: 
1.486 - 1.658 / 0.172 (strong birefringence blink in cabochons)
Pleochroism: 
None to weak.
Dispersion: 
0.017
Magnification: 
3 directional cleavage cracks, very strong doubling of facets and inclusions, iridescent effect at cleavage cracks, fingerprints and crystal inclusions.
U.V. Fluorescence: 
Variable, generally pink under longwave.
Spectrum: 
Not characteristic.
Cause of Color: 
Impurities.
Treatment (Enhancement): 
  • Colorless Impregnation is used incase of a colorless and white fractured or cracked calcite. Generally colorless oil, resin, wax, as suitable, of the similar refractive index (R.I.) is used to disappear the cracks and fractures.
  • Colored Impregnation (dyeing) is used incase of a fractured or cracked colored calcite. Desired colored oil, resin, wax of the similar refractive index (R.I.) is used to fill up the cracks.
Due to high difference in R.I. range of calcite, the similar R.I. of oil, resin, wax that has been used in impregnation will not help to disappear the cracks or fractures completely.
Specific Tests: 
Effervesces with cold hydrochloric acid.
Synthesis: 
None.
Simulants (with separation tests): 
  • Banded chalcedony for onyx marble (R.I., birefringence)
  • Transparent calcite is a simulant for other natural gemstones like yellow beryl, sapphire, etc.
Geological Occurrence: 
Sedimentary limestone deposits (stalactites, stalagmites) veins, ore deposits.
Sources: 
England, Mexico, India, U.S.A. (California, Utah), Spain, Namibia, Russia.
Cuts & Uses: 
Facetted cuts, cabochon, carving, etc.