Cobalt Star

The Cobalt Star is the power source for Professor E. Gadd's time machine in Mario & Luigi: Partners in Time. The Cobalt Star was shattered into multiple shards by Princess Peach after she used it to trap the Elder Princess Shroob, the leader of the Shroobs that were invading the Mushroom Kingdom of the past. However, her sister, the younger Princess Shroob kidnapped Princess Peach in the castle of the past, which is later turned into Shroob Castle. Destroying the star also caused time holes to appear between the past and present. Meanwhile, Baby Bowser collected the shards of the Cobalt Star as his personal treasure, going great lengths to protect it. With the help of their infant selves, Mario and Luigi go about retrieving the shards and reassembling the Cobalt Star to fix the fabric of time and save Peach from the aliens. They find the first one in Bowser's Castle, although this causes Baby Bowser to pursue them. After finding the second shard in the Vim Factory, Baby Bowser steals the shards and flees to Yoshi's Island, where the Mario Bros. give chase and retrieve the shards from him. Later in the game, at Star Hill, the "spirit of the Cobalt Star", which is the Elder Princess Shroob, sends the heroes to the Star Shrine, where there's another Cobalt Star shard.

Baby Luigi and Kylie Koopa both realize something is suspicious about the Cobalt Star, with Baby Luigi crying because he might realize that there is a monster inside, but the whole truth is not revealed until it is too late: all of the shards are assembled at Shroob Castle when Baby Bowser steals the last one from Princess Peach. Baby Bowser is transformed into a Shroob mushroom, and the Elder Princess Shroob then uses her regained power to break free of her prison and attack the heroes.

It appears that the Cobalt Star does not exist anymore in the Mario universe, as the Elder Princess Shroob states that the star "is no more".

When the Cobalt Star shard goes near the Junior Shrooboid, the sound that a Shroob saucer makes is heard.

Cobalt Star shard locations
Below is a list of where the six shards of the Cobalt Star are located.
 * In the Treasure Chest Block of the past Bowser's Castle. Until meeting with Professor E. Gadd, this is treated as a normal key item with the name Sparkling Shard.
 * In the possession of Swiggler in the Vim Factory.
 * Inside of Petey Piranha in the Gritzy Caves.
 * At the Star Shrine.
 * In the possession of Toadbert and then Elder Shrooboid at the Star Shrine.
 * In the possession of Princess Peach at Shroob Castle.

Cobalt Star shard
Kobalt-Stern Stella Cobalto