grind·stone 
/ˈgraɪnˌstoʊn/ noun
plural grind·stones /ˈgraɪnˌstoʊn/ noun [count] : a stone disc that can be turned like a wheel and that is used for sharpening tools, smoothing rough edges, etc.
nose to the grindstone
◊If your nose is to the grindstone, you are doing hard, continuous work.
▪ You'll do well at school if you just keep your nose to the grindstone.
▪ You'll do well at school if you just keep your nose to the grindstone.
