A grindstone is a big heavy wheel made of stone. You turn it, and you hold a tool or a knife against it so the rough stone rubs the metal and makes a new sharp edge. Smiths, farmers, and kitchen workers used one for centuries; you may still see a small one in an old workshop. It is a concrete thing, not a fancy word. People also keep the picture in an idiom: if you keep your nose to the grindstone, you work hard and do not look up, as if your face were close to that spinning stone. That does not mean the grindstone itself is “hard work”; the stone is still the stone. You grind on a grindstone; you do not “grindstone” something as a verb in ordinary talk.