Calcite
Calcite var. Iceland Spar (Optical Calcite) · Mineral Specimen / Decorative Stone
Crystal System
Trigonal (Rhombohedral system); Rhombohedral habit with distinct rhombohedral cleavage fragments
Mineral Group
Carbonates (Calcite Group)
Chemical Formula
CaCO3
Formation Process
Sedimentary (precipitation from calcium-rich solutions) or Hydrothermal (deposition in veins and cavities)
About This Crystal
A colorless to yellowish translucent-to-transparent rhombohedron. It exhibits a vitreous luster and characteristic parallel stepped growth or cleavage planes on the surface. The specimen shows visible rainbow 'fire' or internal reflections from cleavage cracks.
Physical Characteristics
Color: Colorless/Yellow; Streak: White; Cleavage: Perfect rhombohedral in 3 directions; Fracture: Conchoidal (rarely seen due to perfect cleavage); Density: 2.71 g/cm3; Fluorescence: Often fluoresces red, blue, or pink under UV light.
Optical Properties
Extremely high double refraction (birefringence of 0.172); Refractive index 1.486-1.658; Exhibits strong doubling of images when viewed through the crystal.
Hardness & Durability
Origin Region
Notable sources include Iceland, Mexico (Chihuahua), USA (Tennessee), and Brazil; found in hydrothermal veins and sedimentary environments
Hardness & Durability
Mohs hardness of 3 (soft); brittle tenacity; poor durability for jewelry due to extreme cleavage and low hardness.
Care & Maintenance
Clean only with warm soapy water; do not use ultrasonic or steam cleaners. Avoid all acids (even vinegar), which will cause effervescence and dissolve the stone. Store separately to avoid scratches.
Rarity & Value
Common mineral, but high-clarity optical rhombs are valued for educational and collector purposes. Value is generally low ($5-$50 depending on size and clarity). No common treatments.
Special Characteristics
Strong Birefringence (double refraction); Rhombohedral cleavage; Effervesces (fizzes) in contact with cold dilute Hydrochloric acid; internal rainbows from light diffraction on cleavage planes.
Lore & History
Historically used by Vikings as a 'Sunstone' for navigation (relying on its polarizing properties to locate the sun on cloudy days). Metaphysically associated with the Crown Chakra and believed to amplify energy and clear blockages.