4.7mm JGS2 optical quartz, optical precision, optical polish.
This means it is about as pure, perfectly shaped, and perfectly smooth as possible. Which is extremely revelant.
Designed specifically for optimal use in our Halo/Nimbus dishes, the quartz Comet also fits most other things.
Please do not use this or any moving object in nails without lubrication. Using more than one sphere can risk them sliding/scraping instead of rolling, in which case damage can occur.