First photo shows the horn bead I received for inspection, along side one from my collection (the larger bead on the left - about 30 mm diameter.) My bead was sold as horn but I would bet that some people would confuse it with an old, darkened phenolic bead if they only have a picture to go on.

The second photo shows the fracture surface of the "mystery" horn bead. I should mention that it was sent from Thailand.

Thanks for all the comments & I hope this was a good illustration of how difficult it can be to ID from a photo!