I have a Kensington Expert Mouse Trackball that I use with mt computer. It originally came with three 2mm ruby bearings that the Large Trackball rides against. (Bearings). During cleaning, I managed to loose one of the 2mm ruby bearings. I have tried replacements made from glass, acrylic, and stainless steel, and they wear out (flat spots form). I decided I needed ssomething that had the same or greater hardness as the rubies that came with the Trackball. I purchased 2mm Alumina Oxide 2mm balls, which have a hardness of 9, similar to the rubies, and so far they are working perfect. The manufacturer of the trackball does not sell replacement ruby 2mm ball bearings, and a new trackball costs around $100 USD. The 2mm Alumina Oxide balls are not showing any signs of wear. This is a good alternative if you should loose one of the OEM ruby ball bearings.
