Dighton Rock Museum; photo by Aaron Knox on Flickr (noncommercial use permitted with attribution / share alike).
Official State Explorer Rock of Massachusetts
Massachusetts designated Dighton Rock as the official explorer rock in 1983. All State Rocks
Dighton Rock is an 11-foot-high "glacial erratic" boulder that once rested on the shore of the Taunton River. Dighton Rock is covered with mysterious petroglyphs (carved designs of ancient and uncertain origin). The rock is now installed in a small museum which is open by appointment (adjacent to Dighton Rock State Park).
Massachusetts also recognizes an official state rock (roxbury puddingstone), a building stone (granite), a glacial rock (rolling rock), a historical rock (Plymouth rock), a state mineral (babingtonite), and a state gem (rhodonite).