In 1928 this quarry in Albany Township produced what is still spoken of as the largest beryl crystal ever found: a hexagonal column reported at roughly 18 feet long and several feet across, weighing tons. Maine pegmatite country made its name on exactly this kind of thing. The quarry is on PRIVATE land and has been closed and reopened to collectors over the years depending on the owner, so ask first; the Bethel and Oxford County mineral shops are the reliable way to hold Maine beryl.
Beryl, feldspar
See it on the map
Other places on the map within a couple of hours.