What wood for the hardie block? If it comes from a tree and is not balsa wood then it will be good!
It's just a piece of wood, what does it matter what tree it comes from? It really drives me mad when I see it written that it has to come from a certain type of tree, why? Does it mean you should not use that piece cut from a tree that came down in the wood? How about the off cut from a fence post that you've been offered? You can use them, it can be difficult enough to get something like that so use what you can.
Don't lose time trying to get the right wood. If it's hard enough to set the hardie into then use it, don't waste time, get the wood, sink the hardie, get cutting!
Ok rant over ;-)