vegetable ivory

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hard ivorylike material obtained from the ivory nut and used in making small objects such as buttons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The nutlike seed of the South American tagua palm; used to make small items such as buttons.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun nutlike seed of a South American palm; the hard white shell takes a high polish and is used for e.g. buttons