A Ghostly Gem is a gemstone covered in dark energy, and looks very similar to a Dark Moon Piece, but has the shape of a diamond. In Luigi's Mansion 3: Haunted Invasion, 10 of them are needed to access King Boo's castle. When you collect all 10 of them, they all stick together and transform into a key.

The first image shows all the 10 Ghostly Gems combined.

The second image shows a Ghostly Gem.

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Ghostly Gem key

The Ghostly Gems when combined together

Nintendo Drilly.

Ghostly Gem

A Ghostly Gem

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