borkő

While we were rolling the barrels back into position, before siphoning the wine back in, I heard a noise like there would be a fragment of metal rolling around inside the barrel.  Sanyi said it was "borkő".  Although I didn't know exactly what this is, I got the idea, since it literally translates as "wine-stone" (and I knew vizkő - which literally translates to water-stone - means calcium).  So I guessed it must mean something that crystallizes from the wine.

I looked it up later and it means "tartar" in English, used in cooking as "cream of tartar". Technically, Potassium Bitartrate. This crystallises in wine casks during the fermentation of grape juice.  It is a product of Tartaric Acid which is the most plentiful acid found in grapes. This chemical is relatively rare in other fruits and is one of the keys to the suitability of grapes for winemaking.

You can sometimes find it in bottles too.