The other thing I think about, is sometimes even the silica in some of this stuff varies in hardness over a range. Some mineral forms of it aren't quite up to cutting hardened steel, while others are. I got to thinking about this again last night, trying different types of silica-based rocks I've collected around here. Quartz is relatively hard, being able to scratch glass at least, while a couple others I tried didn't even do that. I've collected a few pieces of obsidian as well, which is nearly pure silica, save for impurities coloring it. Obsidian, being almost pure silica, is also 'amorphous', meaning it has no crystal structure (like quartz does). That also limits how hard, strong or tough it can be (obsidian wouldn't scratch the plate glass, when I tried it). The crystal structure of different types of silicate minerals makes a difference in how hard they might be, or in how they're shaped (sharp, acutely-angled edges or more blocky or blunt).
With the mix of varying types of silica in garden-variety (sometimes inexpensive) ceramic kitchenware, and with some of it likely not able to cut steel effectively, but still hard enough to do damage to a blade's edge, I think there's a lot of give & take in terms of how effective these ceramics can be. Maybe a little bit of hard silica & alumina to do some cutting. But at the same time, there's other stuff in the mix which does more damage to a keen edge (by plastic deformation or chipping) than good. In other words, while some of the cleaner-cutting elements might be cutting or thinning an edge, some of the other stuff is basically inflicting blunt-force trauma against the keen edge, denting, rolling, chipping or burring it. I suspect that's part of what makes it 'hit-or-miss' in how well some of these ceramics might work for sharpening purposes.
So, all this being said, sometimes you might find something that works fairly well at a given moment, with a given blade. And at other times, not so much.
