Answer:
Take one out and scratch my head I am now black but once was red.
With pointed fangs it sits in wait. With piercing force it doles out fate, over bloodless victims proclaiming its might. Eternally joining in a single bite.
A certain wizard resided in a city made from this gemstone.
It doesn't live within a house, nor does it live without. Most will use it when they come in, and again when they go out.
Who is it that rows quickly with four oars, but never comes out from under his own roof?
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.