Casio licensed a toymaker to shrink its famous F-91W "terrorist watch" (and various other classic models, including the Databank) to fit on your finger. Matthew Burgos at Designboom writes that the Casio Watch Ring Collection is to be found in vending machines and toy stores in Japan–which is to say if you want one in the West, your best bet is likely eBay or Zenmarket.
On July 19 in Japan, Casio is releasing the "Casio Watch Ring Collection" in partnership with toymaker Stasto Stand Stones. The pieces, to be sold individually in capsule machines for 400 yen each, are mini replicas of popular classic and modern Casio watches that are wearable as a ring in size 18 (with 18.7mm diameter). The five model types include the best-selling digital F-91W-1, the DBC-611-1 calculator watch, the world time-equipped AE-1200WH-1A, the analog MQ-24-7B2, and the analog-digital AQ-230GA-9. The displays of the digital models are made with a three-layer structure and the analog models have a two-layer structure for a realistic appearance, but the watches are non-functional.
I like how absurd it looks, just completely ridiculous!