Duotone Balls have been around for a long time and for good reason.
Used internally during masturbation or oral sex these balls will increase the sensations and stimulate the vagina.
They work by having a smaller heavy ball inside that wobbles around with the movement of the vaginal muscles and your own body movements.

Size - Metric
35mm x 35mm

Size - Imperial
1.3" x 1.3"

Power
None Required