Verse 17 of Chapter 13:

 

He sends down water from the sky, and the valleys flow according to their measure, but the flood bears away the foam that mounts up to the surface - and (also) from that (ore) which they heat in the fire in order to make ornaments or utensils, rises a foam like unto it, thus does Allah (by parables) show forth truth and falsehood. Then, as for the foam it passes away as scum upon the banks, while that which is for the good of mankind remains in the earth. Thus Allah sets forth parables.