Calcite is an abundant calcium-containing carbonate, which is noted within industry as an important ore for the metal and as a key ingredient within important household chemicals such as paints and fertilizers. Due to its extreme softness (3.0) and, like all carbonates, vulnerability to attack from even the weakest of acids, the mineral lacks the durability usually required for jewelry wear and is instead usually encountered as a collectors’ mineral. In addition, calcite comprises the, often unwanted, whitish regions within lapis lazuli.

Aside from its very occasional use as a gemstone, calcite is perhaps best known to gemologists as the key optical component within many gemological dichroscopes. These devices make use of the mineral’s inherent ability to split incident light into its two constituent rays, and these are subsequently able to individually display the two or three hues that emanate from the pleochroic gem material under test.