I found old billiard balls. They were dull and dirty and I could not guess of what material they were made. Two balls showed superfine cracks, one crack around the ball and the others starting and stopping at that around crack.
I decided to split a ball holding a chopping-knife on the crack and giving it a firm hit with the hammer.
The result is amazing, the around crack is 5 mm deep and darkened inside. The other cracks are not so darken. The centre of the ball is hard, glossy white, 100% pure cast phenol formaldehyde. The billiard balls have a diameter of 45 mm.