Aluminum Oxide:

Color: Maroon or brown

A/O is formed by combining bauxite and other materials by firing them in an electric furnace.  The resultant mass is then crushed, and the pieces sifted through successively finer screens to assign a grit size (CAMI grading).  When crushed, the resulting pieces are naturally pyramidal in shape.  Due to the shape and the strength of the materials used in it’s making, A/O is a very durable grain.  The grain is worn down during use, sanding finer the longer it’s used. So theoretically, you can start at 80 grit, and after you’ve worked the abrasive for a while, you’ll be sanding more like 100 or 120 grit.  Users take advantage of this characteristic by using the belts on more than one machine or application.  For instance if you have two wide belt machines doing intermediate and finish sanding respectively, you may use a 120 belt on the first machine to do your intermediate sanding, and when the cut for that application is no longer as sharp, the belt can be moved to the 150 grit machine and used there till the cut for that application is no longer sufficient.  This ensures you get the most production and the full value for the belt you purchased.  

A/O is very tough and durable, as stated above, and is used on bare wood, most metals (especially the steels), leather and a wide variety of other materials.  It is a great general purposes grain and probably the most, common grain seen today.