Many people think they have ivory and bamboo sets when what they really have are bone and bamboo sets. Ivory was in very short supply in China in the 1920s, and China was a poor country then. Only a few sets would have ever been made of ivory, and many of these would have been made for the export market, as could be seen with Western indices on the tiles instead of just Chinese characters and numbers. You will see clearly in the photograph what characterizes it as ivory: the crosshatching. Be sure to really study the close up photo of the wind indicator.
Bone does not have the cross-hatching so evident here. Sometimes you will have to look at a piece from several different angles to see the cross-hatching.
French Ivory was designed to mimic ivory. It was made by layering two slightly different colors of plastic, one on top of the other, then slicing through them, as seen here below:
You can see the lines do not intersect but basically run parallel to each other.
Thanks to Katherine Hartman for the use of her photographs.