Scrimshaw crafted before 1989 (elephant) or before 1973 (sperm whale ivory, walrus ivory, etc.) is legal. It is prohibited after those years for commercial import in the USA. Ancient ivory, such as mammoth or fossilized walrus ivory, is unrestricted in sale or possession.
The origin of the word “scrimshaw” is somewhat obscure, but most researchers agree that it probably came from the Old Dutch word skrimshank which means “a waste of time” – as scrimshaw was enjoyed by seamen as the occupation to kill time.
The simplest way is to examine your scrimshaw with a magnifying glass. Plastic items will look completely smooth, while real bone will have a grain.