I would have to disagree when you say there is rarely a nerve hole (When I say nerve hole I mean the center of the tusk. There is rarely an actual hole). If my website can be used as an unbiased sample of chips, well more than half, maybe 70% or more display the center of the tusk.

Take a look at a sample page:

http://www.oldpokerchips.com/IVORY/IN5swithleaves.htm

If the scans are too small, click on any chip for a blowup. One chip has a drilled out center, leaving an 1/8" hole. Another has a drilled out center that has been filled (like a dentist fills a tooth). Some the nerve is barely visible but is clearly there.