Calcite

Calcite is one of the most widespread minerals on Earth and the main constituent of limestone and marble. It is calcium carbonate, the same chemistry as aragonite but with a different crystal structure.

Calcite is famous for its huge variety of crystal forms (over a thousand are known) and its full rainbow of colors, from water-clear Iceland spar to orange, blue, green, and pink varieties. Clear calcite shows dramatic double refraction, splitting an image into two.

It is a defining mineral of hardness 3 on the Mohs scale and reacts vigorously with dilute acid.

Calcite forms in an enormous range of environments. It precipitates from marine and fresh water to build limestone, and recrystallizes under heat and pressure into marble. It also grows in caves as stalactites, stalagmites, and flowstone.

It forms in hydrothermal veins, as a cement in sedimentary rocks, around hot springs (travertine), and as the shells and skeletons of countless marine organisms. This versatility makes it ubiquitous worldwide.

Notable specimen sources include Mexico (orange and honey calcite), Iceland (optical Iceland spar, historically), England, the United States, and many other countries.

The key tests for calcite are its hardness of 3 (scratched easily by a knife) and its strong fizz when a drop of dilute hydrochloric acid is applied, releasing carbon dioxide. It has perfect rhombohedral cleavage, so broken pieces form slanted rhomb shapes.

Clear calcite shows obvious double refraction: lines viewed through it appear doubled. Luster is vitreous, and the streak is white.

Look-alikes include aragonite (same chemistry, different crystal habit and cleavage), dolomite (fizzes only in warm or powdered acid), and quartz (much harder, no acid reaction, no rhombic cleavage).

Calcite, as limestone and marble, is fundamental to construction, cement, and lime production. Crushed limestone is used in agriculture, steelmaking, and as aggregate; marble is a prized building and sculpting stone.

Clear Iceland spar was historically vital for polarizing optical instruments. Calcite is also used in chemical industries, as a soil conditioner, and as a colorful specimen for collectors.

Metaphysically, different colors of calcite are linked to different energies (orange for creativity, blue for calm), though these are spiritual beliefs. Specimen value depends on color, clarity, crystal form, and fluorescence.