Ivory dust is lethal. It's gets in your lungs and NEVER goes away. Much worse than pearl dust. I always figured if I could smell it I was likely doing damage to my lungs.
BTW, it's not a tooth, it's a tusk. Small matter but hippos have both.
Weary: Mastodon ivory is pretty old, 40,000 years or so, so you're pretty safe. It's considered "fossil" but it's not fossilized, only mineralized. Mastadon, wooly mammoth, and walrus of the "fossil" variety are all OK to use. I like to avoid anything that might be mistaken for fresh (elephant, whale, hippo) except for boar which is plentiful and considered a nuisance by most here. There have been a lot of techniques used to make new ivory look old. One of the biggest problems in using ivory of any kind in instruments is dealing with the custom officers. Ivory, no matter what kind, raises a big red flag with them, unless you have documentation. Horror stories abound. If you use it, keep it at home.
The CITES Treaty was signed in 1973, the hippo added to the list (appendix 2) 10 or 15 years ago, I can't remember for sure. All I'm saying is to be careful.