the form factor is less important for the core capability, but does have tradeoffs on convenience, attraction. Make it a dull grey metal to reduce the likelihood of having your hand chopped off
Discussion
Here some pie in the sky idea... You have a NFC ring that is trained on your bio-rhythm. If there's a severe change then it wont sign.
Even crazier, what if starting your car or unlocking your front door took a nsec signing..