The first thing to consider is the condition of your size reduction equipment and the quality of the feedstock. Inspect your feeder, extension hopper, pulverizer, transfer piping, and collection system. Confirm that your equipment is clean, properly maintained, and operating correctly. Discoloration is often the result of metallic contamination, usually iron, from mill liners and piping, which is the result of erosion by the product sliding over metal parts at high velocity. This erosion is normally only seen when the material being milled has a Mohs hardness of five or more. This contamination by softer products can be eliminated by using nonmetallic liners, such as polyurethane or UHMW polyethylene. The higher Mohs hardness products, such as quartz, may require harder liners ranging from metals like hardened tool steels to sintered tungsten carbide, or various ceramic linings like alumina or silicon carbide.