hard as a rock

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*hard as a rock

 and *hard as stone
very hard. (*Also: as ∼.) This cake is as hard as a rock! I can't drive a nail into this wood. It's hard as stone.
See also: hard, rock
References in classic literature ?
In all matters affecting income, the sales of timber, wheat, and wool, the letting of lands, Vronsky was hard as a rock, and knew well how to keep up prices.