According to the etymologies the Latin calx had two meanings, leading to separate English words. On the one hand it meant a "pebble" or "small stone," regardless of the type of stone involved.(Hence calculate, from Latin calculus, small stone used in reckoning, diminutive of calx, calc-, small stone for gaming. See too calx.) On the other hand, it meant "limestone," whether that limestone was in the form of a pebble or a larger chunk. (Hence calcium and chalk, each from Latin calx, calc-, lime.)

It seems strange that the same word would have meant both a particular size of stone, and a particular type of stone. Is AHD accurate and, if so, how did this come about?

None of the dictionaries I consulted gave more than one entry for calx 'gaming piece; limestone'. (There is, of course, another word calx which means 'heel'.) Other words for stone: saxum and lapis; other words for pebble: lapillus and calculus.

I have this vision of the compiler of the first Latin dictionary asking a local, "What sort of stone is that?" On getting the reply "That's a pebble." he diligently wrote down "limestone = pebble".

I'm probably wrong, though.

Here's a money-making idea for you. Devise a little handheld electronic gizmo which would determine just the right amount of powder to apply to a baby's bottom for maximum comfort.

The device would be called, of course, a talculator.

Wordnerd, it is coincidental that you should post about the etymology of "calculus." When Richard, arnie, Shu and I were on a tour of a Young's Brewery while in England, they showed us some limestone and said that's where the word "calculus" originated. So, is it true? I thought not.

I've tracked it through Perseus, and I'm going to guess the shift of meaning came from the art of concrete-making.

The original word is the Greek khalix 'pebble', thus also 'gravel, rubble' used in building. I don't think the Greeks knew concrete -- Strabo was in the Roman period when he referred to khalix being used to make concrete.

So the Romans borrow this word as calx -- an early borrowing, as you can tell by the simplified form, not *chalix. They have no need for a word for 'pebble', they've got plenty of them lying around, so it comes in as a technical term, 'limestone', or 'lime'.

It was also used to mean 'gaming-stone', but the more usual word for this was the diminutive calculus. The plain calx basically meant 'lime', and thus also 'finishing line' and post-classically 'the end'.

I found a good explanation of Roman concrete.

It was calculus, a diminutive created in Latin, that took on the 'small stone' meaning, and specifically small stones used for operations such as gaming and reckoning, and also kidney-stones. So calculus also meant 'computation' in Latin.