I think you would have noticed a gallstone, but anything's possible. Could also have been a kidney stone, but it would have been small not to have hurt. If it was in fact some sort of internal stone you passed, they are in fact badly absorbed calcium, though the reasons for forming them can be quite variable. Often, it's too much oxylate, but some think it can be any badly absorbed calcium, even calcium carbonate, which is itself a rock -- think oyster shells and dolomite (the latter really is a rock). Some people can break this apart and absorb it, but many can't, and it has to go somewhere. So it isn't so much a case of leached calcium -- that would pass through the system without forming a stone -- as unabsorbed calcium. But gallstones and kidney stones are a bit more complicated, as they often indicate something wrong with those organ systems as well. There are also some medications that form stones as a side effect, though I think usually, again, in the kidneys.