every gallstone I have seen sinks. anything that froms a precipitate sinks doesn't it - that's a rule of chemistry. and if it sinks in bile (which is denser than water) it must sink in water. get some Gallstones yourself and see - if not read the texts and see what they say. it's all out there if you look.
occasionally cholesterol stones do float in bile but I don't know that they would float in water - in any case that is the exception rather than the rule.