Green’s Dictionary of Slang

(US black) a tough person, both emotionally and physically.

1956		‘Ed Lacy’ Men from the Boys (1967) 28: What’s all this about [...] upset stomach? According to my records you’re a typical hard rock.
1961		J. Scarne Complete Guide to Gambling 681: Hard rock – 2. A gambler who refuses to lend money. 3. A player who is hard to beat.
1968		T. Wolfe Electric Kool-Aid Acid Test (1969) 64: The hard rocks are hunkered down telling jokes about pussy.
1988		Ice-T ‘Heartbeat’ [lyrics] That’s who I made it for, the hardrocks.
1992		S.L. Hills Tragic Magic 145: I’ve seen a change from the James Cagney days when you had to be hard rocks.
1995		L. Stavsky et al. A2Z.