Back in March, 2016 at GaryCon 8, +Satine Phoenix gave a bunch of us these d20 rings, that you can wear and roll a d20. Very cool.
They are CritSuccess rings.
They take a bit of working the grit out, dish soap & warm water work well. Once you have them spinning freely, they seem to generate random numbers.
It is a cool trinket for those of us who collect dice and other game memorabilia.
I can see using them for a DM roll of a d20, if it needed to be secret.
They also have rings for other single dice and multi-dice combinations like 3d6. If you really like a ring or two on every finger, this might be for you.