Several common abrasives grain in coated abrasive

Ceramic Alumina – Ceramic abrasives provide the longest life & fastest cut rate of all coated abrasives. They are the newest grain innovation and represent the ultimate in performance, providing an aggressive, consistent cut under moderate to high pressure. Ceramic is usually red or orange in color. Used primarily on metal applications.  Ceramic is available in grits 24-120.

Zirconia Alumina– Zirconia Alumina is commonly referred to as Zirc or Zirconium.  Zirc is ideal for coarse to medium grit applications on metal (best choice for abusive applications).  Zirc is usually green or blue in color.  Works best under high pressure (which is required for the grain to fracture exposing new sharp edges).  It has large fracture planes and it self-sharpens as it cuts.  Zirc is available in grits 24-180.

Aluminum Oxide– Aluminum Oxide is the most common & least expensive. The good starting point for most metal and wood applications.  Aluminum Oxide is usually brown or reddish in color, but can be blue, green, or yellow (which usually indicates the presence of a grinding aid/lubricant). It is durable with tough cutting edges, but it dulls during use.  Aluminum Oxide is available in grits 24-600.