calx

Latin calx (genitive calcis) meant limestone, and by extension, the calcium oxide (quickite) produced by burning limestone in kilns. The Romans burned enormous quantities of calx to make morite—Roman concrete, the material that built the Pantheon, the Colosseum, and aqueducts that still stand. The word was industrial before it was scientific.

When mineralogists in the eighteenth century needed a name for crystallized calcium carbonate—the mineral that forms stalagmites, seashells, marble, and chalk—they derived calcite from the Latin root. In 1808, Humphry Davy isolated the metallic element from lime and named it calcium, from the same calx. One Latin word gave birth to a mineral name, an element name, and a verb (to calcify).

Calcite is the most common carbonate mineral on earth and the primary component of limestone and marble. The White Cliffs of Dover, visible from France on a clear day and serving for centuries as the first sight of England for arriving travelers, are composed largely of calcite from the shells of ancient coccolithophores—single-celled marine algae that lived and died by the trillions in a Cretaceous sea.

Iceland spar, a transparent variety of calcite, displays double refraction: objects viewed through it appear doubled. Viking navigators may have used Iceland spar as a sunstone to locate the sun on overcast days by detecting polarized light. If true, calcite helped the Norse discover North America five centuries before Columbus.