From Longman Dictionary of Contemporary English(as) solid/steady as a rock(as) solid/steady as a rocka) STRONG OBJECTvery strongly built or well supported and not likely to break or fall a large sofa, solid as a rock b) STRONG PERSONsomeone who is as solid or steady as a rock is very strong and calm in difficult situations and you can depend on them → rock-solid → rock
Examples from the Corpus
(as) solid/steady as a rock• It was as solid as rock.• Peter was as steady as a rock, keeping the ball on the fairway and hitting nearly every green in regulation.• It was simply not admissible that something as blatantly solid as a rock could have come from the heavens.• Skipper Alan Kernaghan again led by example, with Nicky Mohan solid as a rock alongside him.• The door was solid as rock. 