Is it ensure, insure, or assure?
early 13c., from Old French gravele "sand, gravel," diminutive of grave "sand, seashore" (Modern French grève), possibly from Celtic *gravo- (cf. Welsh gro "coarse gravel," Breton grouan, Cornish grow "gravel"), perhaps ultimately from PIE *ghreu- "to rub, grind."
gravel grav·el (grāv'əl)
Sandlike concretions of uric acid, calcium oxalate, and mineral salts formed in the passages of the biliary and urinary tracts.