A shard is a colored fragment of an ancient tool that was introduced in Generation III. While they have no use on their own, several NPCs apparently have some use for them, and will trade other items or services for these objects.
Generation IV diversified the use of shards by an order of magnitude, and made them far easier to find than they were in Generation III. In addition to being found in several places on the field, shards are a commonly-found item mining underground in Sinnoh, where their appearance causes some speculation that they are related to plates. In Pokémon Diamond and Pearl, the shards' only use is to be traded ten at a time for the weather-changingTMs on Route 212. In Pokémon Platinum, however, their use is augmented. While the TMs are no longer available by trading ten, three move tutors will accept groups of eight in specific colors to teach moves to a Pokémon. Their availability is also increased, with Mr. Fuego, owner of the Fuego Ironworks, trading one of each color to the player for a Star Piece.
The color of each shard corresponds to a Generation I game.
Underground, the shards have the same palette as several plates, with the Red Shard matching the Flame Plate, the Blue Shard matching the Splash Plate, the Green Shard matching the Meadow Plate, and the Yellow Shard matching the Zap Plate. This causes some to believe that the "ancient item" mentioned in the bag description is in fact one of Arceus's plates.