Hard as a rock.
More Traditional Proverbs:
The best defense is a good offenceTraditional
The eye is the mirror of the soul.
Traditional
The marksman hitteth the target partly by pulling, partly by letting go. The boatsman reached the landing partly by pulling, partly by letting go.
Traditional
Remain hungry but do not start begging.
Traditional
A door must be either shut or open.
Traditional
Each truth you learn will be, for you, as new as if it had never been written.
Traditional