they are catalin (a/k/a bakelite) chips. Condition is important to me -- they are often found scratched and dull. In decent shpe they seem to sell for about 75 cents each. For some reason they go for close to $1 each if in mint condition in their original carboard box.

Here is some info in the subject:
http://www.antiquegamblingchips.com/catlinRk_best.htm

Robert