My invention needs a material that is hard. Ideally, the hard material will be rocks or stone.
However, anything hard will do.
If I mention that rocks or stones are used, then if someone steals my invention by substituting the rocks or stones for something else that is hard, is that infringement?
So instead of mentioning a specific material in my patent, can I just use the term "a material that is hard" so that it becomes more general?
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.
Protection in a patent is determined by the claims. In the body of the patent you can describe the use of rocks or stone along with other acceptable materials. In the claims, you should try to allow the use of any hard material. What you can get allowed in claims is often impacted by prior art so that might present limitations on how broadly a claim can be written. So to answer you question specifically:
If I mention that rocks or stones are used, then if someone steals my invention by substituting the rocks or stones for something else that is hard, is that infringement?
If you have claim that specifies "rocks and stone" then someone else can avoid infringement by substituting a different material. If you have a claim that specifies "hard material" then using a different hard material will still infringe.
So instead of mentioning a specific material in my patent, can I just use the term "a material that is hard" so that it becomes more general?
Whether you can get away with "a material that is hard" in a claim depends on prior art and possibly how well the body of the patent describes the term "hard". Hardness is an engineering term and can be measured in physical units. You may need to describe what "hard" means in physical units such as the Rockwell scale.
In any case, I always suggest working with an actual patent attorney when drafting and prosecuting a patent application. A patent is a legal document. Although it is possible to obtain a patent without a lawyer, the likelihood of doing so is far lower. Also a patent can be weak or strong. A lawyer is much more likely to get you a strong patent.
