No, folks, it's Laguna B-Mix 5 stoneware for cone 5 or 6. Fired to 6. Glazed with Robin's Egg first and Hot Chowder on the rim. And it's my favorite just because I like what it looks like. I don't use it everyday because I prefer larger mugs! And anyway I often just rinse my mugs out and hang them up again, rather than putting them in the dishwasher. Somehow I think it's something else...but I DO appreciate everyone's attempts to explain it!
Some of my mugs have stains inside from coffee, but not this one.
So I think the mystery continues!