Ivory or bone ?

[Berkley]

The regularly alternating dark and light lines are characteristic of the type of celluloid known as “French ivory”, first made in the 1860s and often found in knife handles.

(Shown is a knife with French ivory scales made by George Wostenholm of Sheffield).
Impossible to say what environmental or traumatic effects caused the longitudinal fracture. With as much certainty as possible from merely photographic evidence, I vote “not ivory”.