In 17th century in Italy, a version of abacus was used as a meter in the carriages to calculate the distance traveled. It was an innovative device wherein after a certain number of revolutions of the wheel, a pebble would fall into the lower box of the device. At the end of the journey, the number of pebbles in the lower box would be counted to calculate the distance traveled. The pebbles used for this were typically made of limestone or chalk. Calx is the Latin word for the same. This was the root for the Latin word calculus which meant calculus and reckoning, and also, pebbles.
Incidentally, the word chalk which is derived from the Old English word cealc, has also its root in calx. This path of calx also led to dental term calculus which means tartar i.e. a hard crust of calcium salts and food particles on the teeth. This usage has been traced back to 1732. The root calx, itself goes back to the Greek word khalix which means small pebble which was in use way back in ancient times.
The distict of Chelsea in UK (formerly called Chelchuthe (1300), O.E. Chelchede (1086), Celchyth (789), Caelichyth (767)) is also named so for it was probably here that chalk or limestone was unloaded from Kent, thus, leading to a combination of the Old English roots cealc and hyth (landing place). As per the Online Etymology Dictionary, Chelsea became a female proper noun much later, and gained prominence only from the 1970s. It was amongst the Top 100 names for girls from 1984 to 1998, peeking at No. 15 in 1992, probably something to do with Bill Clinton's acsendedncy to President's office in that year.
Other words from the same root include calcium, crayon, calcarius, calculate. There are other words like calcaneus, calceus which are related to the heel bone which are derived from the Proto-Indo-European root (s)kel which means to bend and led to an entirely different Late Latin word calx.
