BitModel
perfect
A human skull signet ring. The inner diameter of the ring is 20 mm. You can scale the whole model by percentage to make it fit your finger. Remember to measure your finger's joint not the soft part. I've cast the same model in brass, and aluminium.
(In the first five pictures I've added some scratches and rust to the metal in post processing; They are not part of the design.)
Update: A version with thicker walls have been added