What is this fantasy book about magic powder from a tree that lets you see the future?

You are likely referring to _Jonathan Strange & Mr Norrell_ by Susanna Clarke. In the novel, there is a magical object with similar properties; it is actually called "fairy powder," although it comes from an apple tree as a by-product of fairies eating and then excreting the apples. Jonathan Strange uses these fairy droppings mixed with his ink in order to see glimpses of the future.