~~~ with chips such as these is the detection of the "schreger lines" (cross-hatching) in the field and especially along the edge. The central pinpoint dot is the nerve... a sure sign that it is elephant tusk ivory and not a Celluloid-type plastic or animal bone which is kind of close in color, sometimes, in the natural, unpigmented portion of the chip.

An excellent reference in seeing the difference in both is Robert Eiseenstat's antique gambling chips website.