grinding wheel: a wheel composed of abrasive material; used for grinding
wheel: a simple machine consisting of a circular frame with spokes (or a solid disc) that can rotate on a shaft or axle (as in vehicles or other machines)
geastrum coronatum: an earthstar with a bluish spore sac and a purplish brown gleba; at maturity the outer layer splits into rays that bend backward and elevate the spore sac