cealc

Old English cealc comes from Latin calx (limestone, pebble), which was borrowed from Greek khálix (pebble, gravel). The word traveled from Greek to Latin to Old English along with Roman building technology — the Romans used chalk and limestone extensively for construction and for making quicklime (calcium oxide). The chemical symbol Ca for calcium comes from the same Latin calx. Chalk, calcium, calculus, and calculate are all siblings — calculus originally meant a small pebble used for counting on an abacus.

The White Cliffs of Dover are chalk — specifically, Upper Cretaceous chalk deposited between 100 and 66 million years ago. The chalk is composed almost entirely of coccoliths, the microscopic calcium carbonate plates that armored single-celled algae called coccolithophores. When the algae died, their plates sank to the seafloor and accumulated. The Cretaceous period is named from Latin creta (chalk) because of the massive chalk deposits formed during it. The geological period is named after the rock. The rock is named after a Greek pebble.

Blackboard chalk is not actually chalk. It is gypsum (calcium sulfate), pressed into sticks. True chalk is too soft and too dusty for writing. The substitution happened in the nineteenth century, but the name did not change. Teachers have been writing with 'chalk' that is not chalk for over 150 years. The word migrated from one white mineral to another, and no one objected because no one noticed.

Chalk lines, chalk outlines, chalk talk, chalk it up — the word has generated metaphors from its whiteness and impermanence. A 'chalk outline' marks where a body was found. 'Chalk it up to experience' means to accept a loss. 'Chalk and cheese' means two things that look similar but are fundamentally different. The temporary, erasable quality of chalk marks — visible but not permanent — is what makes the word useful as metaphor.