From the Super Mario Wiki, the Mario encyclopedia
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Gem Goobs are rare, shiny, blue ghosts found in The Last Resort in Luigi's Mansion 3. They are Goobs with crystalline bodies that drop gems when defeated. If Luigi finds one, it will panic and run around, trying to escape. They do not take damage while being vacuumed and must be slammed to deplete their health. They have 80-120 HP. Usually, the Gem Goobs hold white gems, but on floors 12 and 13, the ghost holds a different gem. On floor 12, The Spectral Catch, a Gem Goob drops a red gem. On floor 13, the Fitness Center, there are 2 Gem Goobs: one with a white gem and one with a purple gem.
Unlike Golden Goobs, they can be captured anytime upon revisiting the same room where the player first found the Gem Goob, likely due to the fact that all of the Gems are needed to unlock the Suction Shot Type-C.