Answer:
A certain wizard resided in a city made from this gemstone.
Come up and let us go. Go down and here we stay.
Many have heard it, but nobody has ever seen it. It will not speak until spoken to. What is it?
Don't do this in the shallow end.
A harvest sown and reaped on the same day In an unplowed field, Which increases without growing, Remains whole though it is eaten Within and without, Is useless and yet The staple of nations.
What is so fragile that saying its name breaks it?
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?