There are several different types of abrasives used for sandpaper, including:
Aluminum oxide sandpaper
Aluminum Oxide: This abrasive is the most common all-purpose sandpaper material for home or workshop use. Aluminum oxide sandpaper is economical and suitable for most materials – including wood, metal, paint, plastic, and fiberglass. One advantage of aluminum oxide is that it’s friable, meaning the particles fracture to expose new sharp edges as you sand, which makes the sandpaper last longer.
Garnet: An inexpensive sandpaper abrasive, garnet is used mainly on wood. Garnet is a natural material that grinds down smooth with use. This allows garnet sandpaper to produce a velvety surface in fine woodworking projects, though it also tends to wear out rather quickly.
Silicon carbide wet/dry sandpaper
Silicon Carbide: This synthetic abrasive is gray or black in color and often backed with a waterproof material for use in wet sanding. Since it’s harder than many other abrasives, silicon carbide is mainly used for sanding hard materials – such as metal, plastic, or fiberglass – as well as sanding between coats of paint or finish. It’s generally not used for sanding wood, except on wood floor sanding machines.
Ceramic: A very hard abrasive, ceramic is generally used for removing large quantities of wood, such as with a belt sander. Ceramic sandpaper generally doesn’t produce a very smooth surface.
Emery: Used for sanding, polishing, or removing rust from metal. Emery usually has a cloth backing to make it more durable and flexible.