The traditional Ben Wa Balls are small weighted balls made of solid metal.  Today, there are many variations of Ben Wa Balls in different materials such as glass and silicone coated balls.  The glass and other types balls are usually larger than the metal balls since the material is lighter. 

Duotone balls are a modern twist of Ben Wa balls. They are much larger than the traditional balls because Duotone balls contain a smaller weighted ball inside that bounces and rolls around during movement to cause vibrations. They can be made from different materials, such as plastic, silicone, and rubber. The movement is noticeable and causes sexual arousal when used. Duotone balls normally have a string that connects the balls together with a retrieval cord on one end.  This makes it easier to remove them as well as tug on them to cause vibrations. 

Another cousin of the Ben Wa Balls are Kegel Balls.  These are shakers like duotone balls, but instead of a cord connecting them, are encapsulated usually by silicone.  The encapsulated balls are more rigid than duotone balls to perform kegel exercises while the shakers vibrate during movement for stimulation.  Kegel Balls are the largest of the Ben Wa Balls and Duotone balls.