Are you tying in flushing to moon ryhthms? I find that helps get more stones out, bigger ones too. If you need help on dates, let me know. The new moon is especially powerful, plus we have an Equinox coming up soon that will be amazing too.
I have had a few stones that were 1 inch in size come out, I'm a hit and run kind of flusher, so never fish them out for inspection, so don't really know what kind of stones they are ... they just range from either bright green (most of my earlier flushes) to pale and darker/tan browns (most of the later flushes)
I'm sure that pesky stone will come out when it's good and ready :-)