Answer:
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.
A certain wizard resided in a city made from this gemstone.
Gets rid of bad ones, short and tall. Tightens when used, one size fits all.
What lies in a tunnel of darkness. That can only attack when pulled back?
I am the red tongue of the earth, that buries cities.
People take me on walks and I run and have fun. What am I?