Question: What material is used to make Mah Jongg tiles.

Answer: Contemporary mah jongg sets come with tiles made of plastic or resin.  Vintage mah jongg tiles were made of paper, wood, plastic, bone, and ivory. Bone and Ivory are natural materials with indicators on the top and bottom edges of the tiles that differentiate them.  Bone has pits and irregular grooves with discoloration and ivory has cross-hatches.  A tile that has smooth edges is an indicator that it is some kind of plastic.  Ivory tiles are extremely rare. When someone says they have an ivory set, it’s more likely made of bone. If you see consistent faint lines in the tile, it is French Ivory which is made of celluloid. Most vintage tiles are made of Chinese Bakelite, which isn’t actually Bakelite, or Catalin, which is Bakelite. These tiles are not hand carved. If the tiles in an old set are white, pink, or, green, they are not Bakelite. 

Sources:  Real or Repro, differences between materials, Sports Collectors Daily, about plastics, Spruce Crafts, Bakelite vs Catalin, Sloperama, materials used in mah jongg tiles.