According to this excellent 99% Invisible podcast, billiard balls were made of ivory tusk up until they found a suitable replacement in the 1950s. Think it’s easy to make a billiard ball, check out this video of the process.
Polygoon – Hoe biljartballen worden gemaakt from KNBB – Biljart TV on Vimeo.
http://99percentinvisible.org/episode/the-post-billiards-age/