This is a lovely vintage ring in 10K yellow gold set with a square black onyx with a tiny diamond in the center. It is marked 10K PSCO. PSCO is the mark of the Plainville Stock Company. They have been in business since 1874 and are still producing jewelry today. This ring dates to around the early to middle part of the 20th Century. The style could be worn by either a man or a woman, and ti would make a great pinky ring.
It is a size 5 1/2 and it is in very good condition.