If it were ceramic, you'd expect some points where the glaze didn't cover. Glaze is applied as a liquid, which then melts on a second firing of the piece. The usual practice these days is to leave bare the section of the base that sits on the kiln shelf, so the matt, slightly rougher surface of the ceramic shows through. An older practice involved coating the entire piece in glaze, but using some sort of stand, which was snapped off after firing, leaving characteristic marks.