Lately, I have been watching the original Star Trek series--something I have not regularly done since about 1988. I'm watching the show from start to finish. I noticed that on one early episode entitled "Mudd's Women," Dr. Leonard McCoy wears on his left pinky finger a gold ring with a blue stone.
It struck me as an unusual costuming choice, so I did some googling. Terry Lee Rioux's From Sawdust to Stardust: The Biography of DeForest Kelley's, Star Trek's Dr. McCoy explains why Kelley wears a pinky ring.
DeForest Kelly dearly loved his mother, Clora Kelley. Clora owned a ring that her brother had won in a card game while he was in France. When Clora died of cancer in 1957, her son was consumed with grief. But he was private about the depth of his feelings. He asked for only one item from her possessions: the ring. He wore it from then on in remembrance of her.
When Kelley was recruited for Star Trek, Gene Roddenberry was firm: the actors would not wear jewelry. But Kelley was firmer: if he wasn't allowed to wear his ring, he wouldn't be on the show. Roddenberry conceded. You can see Kelley's ring throughout the series, though the stone is often turned into the palm to make it less noticeable.
In the movie Star Trek V: The Final Frontier, Sybok forces McCoy to relive his euthanasia of his own father. DeForest Kelley's ring gleams in the faint light during the scene of mourning and loss. It is a fitting accent to a story of the loss of parents. Kelley would continue to wear it until his own death in 1999.
When Star Trek was rebooted in 2009, Karl Urban took up the role of Dr. McCoy. As you can see in this screenshot, he wears a ring on the pinkie finger of his left hand. According to an internet rumor, he did so to honor his predecessor, DeForest Kelley.