‘I didn’t know you could be a witch in the city,’ said Tiffany. ‘I was told once that you need good rock to grow witches, and everyone says the city is built on slime and mud.’
‘And masonry,’ said Mrs Proust gleefully. ‘Granite and marble, chert and miscellaneous sedimentary deposits, my dear Tiffany. Rocks that once leaped and flowed when the world was born in fire. And do you see the cobbles on the streets? Surely every single one of them, at some time, has had blood on it. Everywhere you look, stone and rock. Everywhere you can’t see, stone and rock! Can you imagine what it feels like to reach down with your bones and feel the living stones? And what did we make from the stone? Palaces, and castles and mausoleums and gravestones, and fine houses, and city walls, oh my! Not just in this city either. The city is built on itself, all the cities that came before. Can you imagine how it feels to lie down on an ancient flagstone and feel the power of the rock buoying you up against the tug of the world? And it’s mine to use, all of it, every stone of it, and that’s where witchcraft begins. The stones have life, and I’m part of it.’

Terry Pratchett "I shall wear midnight"