Zion Canyon in southwestern Utah is one of the most strikingly beautiful places I’ve ever had the pleasure to visit. 150 million years of sedimentary layers were slowly lifted to over 10,000 feet (3,000m) to form what we now refer to as the Colorado Plateau. This gradual elevation change accelerated rivers into fast-moving, stone-cutting water-saws that channeled through the layers of sedimentary rock by carrying silt, sand and rocks downstream – all the while wearing their courses deeper and wider by abrasion. The north fork of the Virgin River wore down the plateau to create Zion Canyon and every year carries three million tons of rock and sand through this spectacular canyon, carving it a little bit deeper year after year. I sometimes think about this when I’m sharpening.
The Virgin River does its honing with whatever it carries along with the flow; debris made mostly of grit composed of silicon dioxide (sand). We can take a much more aggressive approach to abrading steel than the Virgin River does to the Colorado Plateau by selecting the hardest, sharpest grit particles available. We are able to select the best-suited abrasive for the job. Numerous compounds are used as abrasives: garnet; cerium oxide; cubic boron nitride; chromium oxide; iron oxide; zirconium dioxide; etc. However the ones we use for sharpening – whether as loose grains or as sandpaper, bench stones or grinding wheels – in the vast majority are some form of silicon dioxide, silicon carbide, aluminum oxide or diamond. Sharpening hardened tool steel requires grit particles that are harder than the steel and sharp enough to dig in and scratch away a bit of the steel.