Bill Tindall, E.Tn.
If I remember my statistics correctly the error in measuring volume of a stone will be cube root of three (1.7) times the error in a single measurement. To distinguish 2.6 from 2.5 requires detection of a 4% difference. The single measurement error must be less than 2%. This accuracy is easily achieved with calipers. But. my stone has rounded edges, How to account for that loss of weight?

If I can find a graduated cylinder the stone will fit in it will be able to measure volume with an accuracy better than 4%. I have no means of measuring submerged weight, but that would be the way to go.