Antique solitaire button with detachable back, which joins with a spring mechanism. The top is covered in (I think) black and white banded agate - or could be black glass with a white line, held in a brass rim. Backstamped W E WILEY & SON - PATENT. Tiny chip (see 10 o'clock position on main photo) and priced accordingly. Still a lovely old collectable.

Size 3/4" (1.8 cm)