Cliché very steady and unmovable; very stable. (*Also: as ~.) His hand was steady as a rock as he made each incision. You must remain as steady as a rock when you are arguing with your supervisor.
Firm, dependable, as in Betty always knows her part; she's steady as a rock. This simile uses rock in the sense of "something that affords a sure support," a usage dating from the early 1500s.