Garnet:
garnet sandpaper is mostly used for porous surfaces such as wood. It will not hold up well for metal sanding.

Emery:
emery is a general purpose sandpaper with a cloth  backing. Emery cloth comes in fine, medium, coarse, and extra coarse.

Aluminum Oxide:
This is the most common sanding product today. Most sanding belts,sheets, and discs are made with aluminum oxide. They will work well for sanding almost any porous surface without clogging. Aluminum oxide will also sand (polish) metal.

Zirconium:
Zirconium is a very tough material. It will sand metal or wood very well but may clog too fast when sanding wood to be practical in fine grits. However, an 80 or 100 grit zirconium belt on a portable sander will make quick work of a woodworking project and the belt will show little wear or clogging.

Silicon Carbide:
This product is designed for sanding very hard materials. It is used to polish metals, stones, sharpen knives, etc. It does clog very fast if used to sand wood etc