What do calculate, calcium, pebbles and abacuses (or abaci, if you prefer) all have in common? The verb calculate comes from the Latin calculāt- participial stem of calculāre to count, reckon, and from calculus a stone. Calculus, for its part, comes from the Latin diminutive calx stone, pebble.
In medicine, calculate, used as an intransitive verb, means “to form stone in the bladder”. Edward Topsell, in his Historie of Foure-Footed Beastes (1607), wrote about “The same‥with Parsly drunke in Wine‥dissolueth the stone in the bladder, and preuenteth all such calculating grauell in time to come.” Calculate, used in this now-obsolete sense, and calculus, which refers to concretions occurring accidentally in the animal body, appear to share a common Latin derivation. In 1627, reference was made in W. Sclater Expos. 2 Thess. to “That flagellum studiosorum, Calculus Renum”, a kidney stone. There are many types of calculi, and they are named for the various parts of the body in which they occur: renal (in the kidneys), vesical (in the bladder), prostatic (in the prostate), intestinal and so on. Calculi can also be named for or from the nature of their composition (e.g., lithic acid, uric acid calculus).
But what does all of this have to do with calcium?
Calcium, one of the “metals of the alkaline earths” and the basis of lime, like calculate and calculus, also traces its etymological root to the Latin calx stone, pebble. Although one of the most widely diffused of all the chemical elements, calcium is only found in composition in nature. It was first separated in 1808 by Sir Humphry Davy, who, in the Philosophical Transactiona of the Royal Society of London stated “I shall venture to denominate the metals from the alkaline earths barium, strontium, calcium, and magnium.”
And what of the connection between stones and mathematics? Calculus, wrote Charles Hutton in his Mathematical and Philosophical Dictionary (1796), is “a certain way of performing mathematical investigations and resolutions”. A calculus, incidentally, also refers to a stone used in reckoning on the abacus or counting board.
So now it all adds up!