Abrasive classification
Abrasives

Quartz: quartz crystals, agglomerated by impurities, form sandstone, which for thousands of years was the only solid abrasive compound used. Today, quartz is only used in certain softwood sanding operations.

Garnet: when it's not pure enough to be used in jewelry, garnet can be used in the manufacture of abrasives for sanding wood.

Emery: composed of a variable proportion of crystallized alumina (35-70%), silica and iron oxide, it is used mainly in the manufacture of millstones, cloth and abrasive paper for manual use; in loose grain form, it is used for polishing.

Natural...