Answer:
A certain wizard resided in a city made from this gemstone.
Without a bridle, or a saddle, across a thing I ride a-straddle. And those I ride, by help of me, though almost blind, are made to see.
Sharp and long, flag of the world. What is it?
What has two spines and a lot of ribs, and carries much but never moves?
What measures out time. Until in time all is smashed to it?
People take me on walks and I run and have fun. What am I?