At some point of time, we have all seen grindstones. And we have rubbed our noses against it also(more so figuratively rather than literally). Grindstones are basically stones used for grinding :).
How do they look? Just like in the picture above/
The dictionary definitions for Grindstone are as follows:
1. A revolving stone disk used for grinding, polishing, or sharpening tools.
2. A millstone.
Idiom based on Grindstone:
Keep / put one’s nose to the grindstone: This idiom means to to work, study, or practice hard and steadily or to cause someone to do so: If I put my nose to the grindstone, I’ll finish the job as soon as a week.
Usage Examples for Grindstone:
1. There’s a gaudy big grindstone down at the mill, and we’ll smouch it, and carve the things on it, and file out the pens and the saw on it, too. -Adventures of Huckleberry Finn by Twain, Mark
2. That the impact of your army may be like a grindstone dashed against an egg–this is effected by the science of weak points and strong. -The Art of War by Sun, Tzu