I would just keep soaking it in fresh S.G. After a few soaks it should stop changing colors. You might want to give it some time between soaks to let the oil migrate back out to the surface of the stone. The effect of Simple Green as far as I can tell is similar to lye - it dissolves the oil, but can't penetrate the stone very far. So oil is removed at and near the surface, then migrates back out again. One of these days I might try lye on an old "don't care" stone.