Talk to Jogo at the Rito Village Inn. His wife, who will give you the sidequest "The Apple of My Eye", is apparently very disappointed in his choice of a honeymoon destination. To make some Baked Apples for her and keep her happy, Jogo needs a piece of Flint.
Give him one piece of Flint and he will give you a silver Rupee worth 100 Rupees. In the future, you can sell him multiple pieces of Flint just like you can sell Baked Apples to his wife, Juney. Here is a table of what he will give you.