Steel does ×2 to Rock

Generations VI to IX, single types. The other way round, Rock against Steel, is ×½.

65 Steel and 74 Rock species among the 1025 this site covers.

Attacker, Grass and Steel Kartana sprite

Kartana #0798

damage×2Generations VI to IX

Defender, Bug and Rock Shuckle sprite

Shuckle #0213

Steel did not exist in Generation I, so Steel against Rock has no value there.

Strongest Steel attackers

Kartana leads Steel at Atk 181

Sturdiest Rock defenders

Dual types that change the answer