Ruby Sharpening Stones. Ruby aluminum oxide is the highest quality aluminum oxide grain and it has a characteristic much like a good quality diamond, when the grain fractures, it creates a new, sharp point ready to do more work. We have found they are the perfect inserts for sharpening skis. Although we say they are 180 grit, the grit number really isn't important or relative because the grit is internalized and not on the exterior of the stone. Normally the grit refers not just to the grain size but it is also represents how high the grit is raised off the face of the stone. We didn't want to produce a sharpening stone with changing grit, so we internalized it so that it always remains the same, no matter how many times you re-fresh the stone with a diamond file (see Maximizing the Life of the Inserts below). In fact, both the grit size and their manufacturing process is proprietary. These stones are meant to sharpen skis, a true alternative to files. The Tuning Stick can demonstrate just how effective the Ruby Sharpening Stones really are, and since they are polishing the edge while sharpening it, fine edge metal finish is maintained.