An abrasive is something that is very hard. It is also rough. It is used to rub things so they get smooth. When abrasives are glued to paper, it makes sandpaper. Aluminium oxide is a common abrasive. Abrasives are used to make the ends of wood pieces smooth and to smooth the cut ends of metal. The hardest natural abrasive is diamond. Diamond is commonly used in saw blades designed to cut tile or stone.