That's not the same thing- coal contains microscopic diamonds but you'll have a hard time scratching sapphire with it. Incidentally, alumina is much closer to sapphire than any pottery. Alumina is the sintered powder form and you need really good quality, high density stuff to be able to break a window. It's also a near-requirement that it has very sharp points.

Alumina will effortlessly scratch gorilla glass, any metal, tungsten carbide and even a lot of sapphire glass. No kind of pottery will do any of those things.

"coal contains microscopic diamonds but you'll have a hard time scratching sapphire with it."

Coal dust will abrade and destroy almost everything and it's because of the graphite, not the diamond. I have a coal mine. It's killed two phones and a drone already.

"Alumina will effortlessly scratch gorilla glass, any metal, tungsten carbide and even a lot of sapphire glass. No kind of pottery will do any of those things."

This is entirely untrue, it depends on the composition of the clay used in making the ceramic, which will be entirely dependent upon the location the clay or clay mix is obtained from. I also happen to do lapidary and pottery work, as a profession (next to LED lighting applications.) You can get clay from Mt. Baldy, make pottery, fire it up, it will scratch the hell out of almost everything because of the sand grain-sized rubies contained within.