A Sunstone Piece is an item you can Collect. You have to take 10 Sunstone Pieces to a Collecting Manager to convert them into 1 Sunstone. The Pieces will disappear after you convert them.
How to Collect: Edit
Go to a Collecting Area, and buy a Collector from a Collecting Manager. Then going into the Collecting Area with your Collector equipped. Then click Start on the box that pops up when you have a Collector equipped. You will then start Collecting.