Most common bone used is cow or buffalo.
There are two main types, bleached and un bleached.
Usually the bone is slightly cooked (boiled) to remove the bits you dont want, then it is bleached if desired.
Personally, I prefer the non bleached. I have found it to be less brittle and also gives a warmer tone.
I have used sun bleached bone, this appears to be less brittle than ammonia bleached, but you usually have to fight the Dogs off whilst trying to steal their bone
I have heard of guys using Roo bone, but it can be difficult to get a Roo bone big enough to cut a decent sized blank from..
Other materials used are Wolly Mammoth Tusk, Elephant Ivory (Grrrrrrrrrrr!), horn and Hippo tooth name a few.