C. '30s looks correct to me. If the rhinestones are not foiled, it is perfectly ok to wash them. If they are foiled, it is ok to clean them. Make sure that you do not leave cleaning solution on them, rinse well, and do not put them away or store them before they are totally dry. I clean vintage silver jewelry as minimally as possible but if the silver has tarnished to the point that it is black, I will wipe it off with a Sunshine Cloth. Some patina is not only acceptable it is desirable.