If you view a gemstone from the top, you will see it has some type of symmetry. Most round brilliant diamonds will have eightfold symmetry; emerald cuts have two or fourfold. In order to cut facets around a stone, the quill must be rotated around its long axis. The index wheel is what allows this to be done in a repeatable, accurate manner. 

The index gear controls the rotation of the gem. They are available in a variety of sizes, but 64 and 96 are the most common. 

Gemworld's Budget Index Wheel with Sticker fits most machines. It features 96 teeth and has a sticker guide you can attach.
Dimensions:
Diameter - 5.2cm
Hole - 1cm
Thickness - 0.5cm