Rashi on Exodus 17:6/רש"י על שמות י״ז:ו׳

1והכית בצור AND THOU SHALT SMITE THE ROCK — It does not say here “thou shalt smite על הצור”, upon the surface of the rock, but בצור, right into the rock — from this it follows that the rod must have been composed of a kind of hard material the name of which is sapphire, and the rock was split by it (Mekhilta).