Kinstones are items found in The Legend of Zelda: The Minish Cap. They are magical stones made by the fey-like Picori that come in four different colors and three different shapes. True to their name, if the two halves of these stones are joined together by two different people, something magical will occur. The stone's color determines both their rarity and power. Green Kinstones are the most common of all the Kinstones, and can be found almost everywhere. Red and Blue ones, however, can only be found in chests or in shops (where in the latter you must purchase it). Gold Kinstones are required to beat the game, and you'll often times go out hunting for them in a certain area.
If you are able to find another character with a Kinstone that is able to fuse together with one of yours, special things with happen if you do so.