First, you go to the place where you find the Bamboo Shoots in spring. If you haven't reached Winter yet, you should always see water. But when it reaches Winter, it shouldn't be filled with water anymore. It will be filled with snow. So you can walk on it. Then you will find a little cave. Go in it and there will be a mine. If you break the rocks, most of the time you will find jewels. Note: This only works in Winter!