(covers information from several alternate timelines)
A lobi crystal was a semiprecious stone used in the production of inexpensive jewelry and ornaments.
In 2371, on Deep Space 9, Quark attempted to sell Harry Kim one of the crystals at an inflated price claiming to have acquired them from a strange creature called the Morn, but Tom Paris warned Harry that a dozen of those crystals could be bought at the Volnar Colony, for only one Cardassian lek. (VOY: "Caretaker")
In an alternate timeline experienced by Kim in 2372, Paris had had a fight with Quark over the crystals, which had ultimately resulted in his parole being revoked and thus Paris never served aboard Voyager. (VOY: "Non Sequitur")
In the video game Star Trek Online lobi crystals are part of a special currency used in-game. These are obtained within Lockboxes and can be used to purchase pets, weapons, costumes and ships unique to the standard ways of obtaining each. These items can be purchased at the Lobi Crystal Consortium, located on Drozana Station. The fact that inexpensive crystals are being used to purchase high-end equipment has seen a few chuckles from players more knowledgeable about their lore.