Most ivory turners probably didn't realize the placement of the ring(s) at the exact same distance from the edge on both sides would create a weak spot for a fracture during rough play.

This photo shows such an example of the fracture and light passing through it... and sometimes a collector/seller would fill in that fracture to where it can be undetected without close inspection... and where the fracture can easily spread later on (such as with glass).

The chip is rare.. and in its original state of preservation has easilly fetched $600... while I have also sold the fractured examples as seen below for $400.