Aluminum Oxide

*Usually Brown,
*Very tough.
*Block Shaped grit that fragments under heat and pressure.  This friability is desirable, because it exposes new sharpe cutting edges.  Usually considered to be the best all-purpose sandpaper for wood working.

Alumina Zirconia

*Usually blue but sometimes green or red.
*It is a combination of Zirconia and Alum. Oxide
*Zirconia is two steps removed from the hardness of diamond.
*This material requires extereme pressure to fracture the grain; therefore, it is combined with alum. oxide to increase fragmentation.
*It maintains a sharp cutting edge and is usually expensive.
*Ideal or fast removal of material.
*Ideal for shaping metal because of it's longevity.

Ceramics

*Usually pink or gray.
*Harder and more durable that Alumina Zirconia and more expensive.
*A favorite among metalworkers.

Garnet

*Orange
*Natural mineral mined mostly in the USA.
*Block shaped grit like alum. oxide but smoother.
*Fragments and dulls faster than A.O.
*Well suited for final sanding because of its smoother cut.
*Tends to close off grain in wood and make stain penetrate more evenly.

Silicon Carbide

*Shiney Black
*When treated with zinc stearate to reduce cloging it will become white or gray.
*Harder, Sharper and more friable than A.O.
*Fragments quickly reducing the potential for loading and exposing new sharp edges.
*Popular in body shops for wet sanding and for no-load sheets.