It's bisqued clay. You can see that there's a chip on the edge, too, from when it rolled onto a concrete floor a couple years ago (shouldn't have made it all round). I've used this on every piece I've made since somewhere between five and six years ago, minus a few because I lose it every now and then. I know most potters won't sign their work until it's done (superstition, or something), but I usually sign mine first because they're too big and delicate to pick them up and stamp the bottom at the end. However, once or twice I've stamped them at the end on the outside toward the bottom. I guess I'm not really consistent at all.
I prefer the stamp over an actual signature in the clay because I think it looks dumb (at least mine does) and also initial stamps are a pottery tradition and I like traditions. All my cool pottery profs have cool initial stamps.
You may note that it says "STK". I know when you have a big initial in the center, that's supposed to be the letter for the last name, but that's just not the way I did it. It doesn't matter anyway because "T" isn't my middle initial. It's just the middle initial I use to sign my art. There is a reason, but it's pretty boring and insignificant. Here it is: my sister and I have a friend who calls her "SB" for Sarah Beth. So I took "ST" because it rhymes with SB, and the T is from Stephanie. Some things (like drawings) I've signed "St. Kellogg" to differentiate from just plain S. Kellogg, which could be either sister. But then it looks like it should be Saint Kellogg, which actually I am fine with. My real middle initial is M. I don't like to use "SMK" because S and M next to each other just makes me think "S&M".