It is unmarked, but tests as 9 kt heavy gold clad. Most antique gold lockets are clad over a stronger, interior metal to give them the heft to withstand being opened & closed frequently.
This locket probably dates from 1875-1920. The hinge & clasp are in excellent condition. All it needs is your mementos! It would make a stunning necklace pendant or could be added to a charm bracelet or watch chain. It measures 1” x 3/4”, and is approximately 1/4” thick.