In the USA, you will need to teach in the description how someone 20 years (give or take) from now will practice your innovation once it becomes public domain upon patent expiration. Your teaching must be specific enough for someone reasonably proficient in the art of your innovation's discipline to put the innovation into practice. I suspect that most patent examiners will reject “is hard” the vague way that you have used it here in this question.
A truck-load of gravel is hard in an entirely different way that, say, a single steel ingot is hard. Let's say that I am trying to patent a driveway. In the description (let alone the claims), I try to get away with “is hard” to vaguely indicate gravel stones without overtly mentioning gravel, stones, small pieces, etc. But is that truly effectively teaching someone 20 years from now to put this innovation of ‘driveway’ into practice. Can someone pick any old hard thing and make a driveway out of it? Can I buy a single steel ingot, plop it down in the mud, try to drive on it, and call it a driveway as per the patent's description and/or the patent's claims? Likely not, and the patent examiner will most likely object to lack of specificity and lack of being in possession of the invention (i.e., lack of knowing specifically what you are talking about instead of painting in broad strokes of generalities to maliciously snare some future rather-unrelated practice as an infringement).
In the description at least, you will need to overtly describe the properties of that which is hard to overcome lack of specificity and lack of being in possession of the invention. If the hard thing is a crucial requisite to the process or apparatus that is being patented (as opposed to just being present non-innovatively), then you must stake your claim of ownership of the hard thing by enumerating its breakthrough innovations in the claims too.