Ben Wa balls
Ben Wa balls (also known as Chinese Meditation Balls) are small, sometimes marble-sized balls, usually made of metal, hollow, and containing a small weight that rolls around. They are used for massage and meditation purposes as well as female masturbation. Some are solid, or may contain clappers or chimes within.
Ben Wa balls are said to originate in Japan where they were called Rin No Tama and were either metal or ivory. Originally they were a single ball placed in the vagina used to enhance the act of sexual intercourse, but shortly evolved into multiple metal covered balls linked by either a chain or silk string for easy removal. Usage of Ben Wa balls create a subtle stimulation, not meant to bring the user to immediate orgasm but rather to tease. Women can "wear" Ben Wa balls all day, or use them while seated in a rocking chair, often to pleasurable effect.
Ben Wa balls may also be used to increase the strength of the pelvic floor muscles, much as Kegel exercises are used today. The effort of keeping them inside will increase muscle tone in the pelvis and vagina which usually increases sexual satisfaction for both partners.