Gray's Grindstone is a nearly mile-long Class IV rapid on the Tuolumne River's 18-mile commercial rafting section between Meral's Pool and Ward's Ferry, known as a favorite among river guides for its challenging entrance that requires precise maneuvering at all water levels.

Located at approximately mile 9. 75 of the run, this rapid can be scouted from either the right or left bank and is typically the most difficult section to navigate, making it a notable feature for both paddle and oar-based rafting trips.