I was set to create a long post about identifying ivory (there are many different types – just ask the tooth fairy) when I found this excellent pdf by CITES.org: http://www.cites.org/sites/default/files/eng/resources/pub/E-Ivory-guide.pdf.
The pdf is in black and white. Here are a couple of full color examples of pre-embargo elephant ivory, mammoth ivory and a “vegetable ivory” cross-cuts showing the Schreger lines in the ivories and the lack of them in the plant alternative (clicking on the image brings it up to full size):
