DoubleFF Swiss ceramic black ball 2.0These bearings are manufactured with 7 balls. The theory behind this is that the slightly smaller diameter of the 7 balls ensures that you can easily get up to speed and that the total weight supported per ball is less than with the "6" ball bearings. The balls are made of black ceramic material, making them even harder than the white ceramic balls and also rust proof. Please note: the shell is made of steel, meaning cleaning is still necessary after