This, it turns out, is is a super rare one—the 21st ball in a c. 1880s set of baseball pocket billiards, which is the only sort of billiards set that contains 21 balls. (Typically billiards and pocket billiards sets contain 15 marked balls plus a shooter.) I believe this ball was designed and produced by George H. Burt of Millburn, NJ. I have found documentation of a few standard sets out there made of clay and engraved in just this manner (with individual balls going for $100+ a pop)—but no documentation anywhere of a baseball pockets billiard set that would have contained the 21 ball. So, rather special—which I love especially for all that “21” represents; I though this would make an especially nice gift for a 21st birthday, to express something along the lines of the whole world being one's to seize!
2 1/4” diameter, numbered on both sides. Both sides documented. Surface cracking and wear to the clay evident, all to the good I think; stable and sound. I believe this once had a band of dyed/painted color like a stripe running around the middle; very faint now but evident as a darker tone.