The Local Hero star came up with the idea of reintroducing jewellery to the Time Lord while visiting the set of Doctor Who origins movie An Adventure in Space and Time, where he met the original sci-fi icon William Hartnell's granddaughter.
She brought along a box full of her grandfather's Who trinkets and the ring he wore on the show caught Capaldi's eye.
He tells TV Guide magazine, "She opened this little box of props from the original series. She showed me this fabulous ring. Later, I had a band made with a stone in it (like Hartnell's). The stone was sent to me by a jeweller whose son wrote this little story that went with it, which was amazingly affecting.
"He said the stone represented and reminded the Doctor of all the the people he's lost in his long life. But it wasn't a sad thing. It was to remember how joyful times could be."