Mercury Star

The Mercury Star is one of the Elemental Stars: small, gem-like items that are each aligned with a particular element. The Mercury Star appears as a small, perfectly round, gleaming blue sphere. It is the source of Mercury Psynergy, which is embodied in water and ice, and was kept in a chamber in the centre of Sol Sanctum with the other Elemental Stars since the sealing of Alchemy. The Mercury Star was one of the Elemental Stars taken by Saturos and Menardi's group shortly before Mt. Aleph's eruption. Saturos and Menardi used the Mercury Star to fire the beacon of Mercury Lighthouse before Isaac and co. could arrive and attempt to stop them.

For the small amount of time it is held for, the Mercury Star is useless in gameplay terms and cannot be dropped.

Trivia
Like all key items, the Mercury Star can be hacked into the inventory using a gameshark or action replay device and be transfered over to the second game although there would be no point to this what-so-ever as it is useless in gameplay terms, takes up a spot in the inventory and cannot be dropped or sold.