Cobalt Shard

From the Super Mario Wiki

Cobalt Shards are items that Mario and Luigi collected in Mario & Luigi: Partners in Time. They were shards, as the name suggests, of the Cobalt Star. Mario, Luigi and their younger counter parts, collected these shards, thinking the whole Cobalt Star would destroy the Shroobs. However, unbeknownst to the quartet, collecting all of them would unleash the Elder Princess Shroob. Mario and Luigi unwittingly collected them all, giving the Elder Princess Shroob all of her power back, at which point she broke out of the Cobalt Star and attacked the Mario Bros.

It is unknown if the Cobalt Star was destroyed, or if it simply shattered into pieces again (although Elder Princess Shroob says, right after being freed, that the Cobalt Star "is no more").