So, it may be as late as mid 20th c. with a signature paying homage to Japan's most celebrated ceramicist, Ogata Kenzan (尾形 乾山, 1663–1743).

To me it appears to have been mounted as a button mid 20th c. But the bead's patina appears to be much older. One of the problems with Japanese art attribution is that it was considered to be perfectly acceptable for a Japanese student to pay homage to highly revered mentors by adding their teacher's signature.

And maybe it really is an eighteenth century Kenzan.