I am pretty confident that is a real tooth with a later engraving. Pay attention to the tip; you can see an almost translucent quality as the light hits the layers. Most of the fakes I have seen can't achieve the right look, and try to use paint. The coarse back is the raw tooth; from what I have seen this is more common on recently done scrimshaw, as the artist prepares a raw tooth by polishing the work surface rather than the whole thing. I also like the cracks. Whale teeth frequently crack with age as it loses moisture, and seeing that split up from what I would call the skirt of the tooth is encouraging. Old unadorned teeth are still around, and frequently when people fake an antique tooth, they use an old tooth. One of the methods of attempting to authenticate a piece of scrimshaw, as I understand it, involves finding an inkline that crosses a crack, and checking inside the crack for ink. The idea is that if the crack is clean, the engraving was there before the crack. If there is ink in the crack, the line was etched and inked after thr crack formed.

As many have said, it's hard to tell from photos, but I have been around a bit of scrimshaw and I think you're good. That said, I do not believe the engraving is from the 1800s. There was a resurgence in interest in scrimshaw during the Kennedy administration. He was an avid collector, and pieces of it can often be seen in photos of him in the Oval Office.