Elephant and rhino tusks are believed to be medicine by some people and are used by some religious practises. But there is no substantiation of this.
The demand is greater than the supply.
The tusk is rare and difficult to obtain.
Yes, because a Mammoth is bigger than one. Mastodons are about the same size as Mammoths.
Valuables are things that are important to an individual like license and phones and stuff that you think are important to you or somebody else.