Fairy Crystals

Fairy Crystals, also known as Magic Crystals, are magical items to turn into Fairy Dust for the fairies, which used them to do magic.

They featured on ABC's Once Upon a Time. They first appear in the "Dreamy" of the Season 1.

Before First Curse
Fairy Crystals grow in the Dwarf Mines, which is ground into Fairy Dust by the dwarves. Dwarves that are newly hatched are instructed by the head dwarf, Bossy, their purpose in the mines. Once the new batch of dwarves is cleaned up, they are handed their own special pickax, which materializes with each of their own personal names. Afterward, they all head into the mines to start their jobs of mining for these diamonds and crushing them into fairy dust for the fairies. ("Dreamy")

A fairy named Nova flies into the mines to turn on a switch and fill her bag with dust. She works herself into a panic when the machine switch jams until a dwarf named Dreamy comes to her aide. She thanks him for his help, but then her bag of fairy dust is scooped up by the assembly line heading for the furnace. Dreamy manages to save the bag in time. ("Dreamy")

After First Curse
Leroy and his fellow dwarves mine for crystals with David in the hopes of making Fairy Dust to rescue Emma and Mary Margaret from the Enchanted Forest. During one day in the mines, Leroy refuses to stop working, even when the other Dwarves want to take a lunch break at Granny's. At one point, he breaks the surface of a wall, which collapses and sends him falling into a cave. From this accidental fall, a whole cavern ceiling of crystals is discovered, to which David, Henry, and Mother Superior are notified. In a celebration of their success, they all head over to the diner to drink and party. ("Child of the Moon")

Afraid that Cora makes her way to Storybrooke, Mr. Gold sides up with Regina to stop her. They go to the mines with a wand and absorb all of the fairy dust there. Later, Mr. Gold uses the wand to cast a spell on the wishing well that will kill anyone trying to cross over from the Enchanted Forest. ("Queen of Hearts")

Since fairy dust is needed for the protection spell Mr. Gold left Belle to work, Mother Superior suggests opening up a vein in the mines so all the magic in the dust will be carried throughout town. Bashful, Dopey, Leroy, Mr. Clark and Walter quarry a large rock containing fairy dust and stop when their efforts yield a crack. Belle hesitates over enacting the spell and asks Mother Superior to do it instead. Mother Superior refuses since the responsibility is for her to handle as Mr. Gold wanted, so Belle follows through with enacting the spell. From the rock, a stream of magic shoots out and raises a barrier over Storybrooke. ("Dark Hollow")

After Third Curse
The Black Fairy finds a growing cache of light fairy crystals in the dwarf tunnels and forms a plan to lure Zelena there and use the witch's magic to turn the crystals into dark crystals so she can start the final battle. Arriving at Zelena's farmhouse, she offers to side with Zelena, noting that she seems alone in the town, though Zelena adamantly refuses. The Black Fairy keeps the offer open, telling Zelena to come to the tunnels if she changes her mind. Instead, Zelena decides, against her sister Regina's wishes, to go there and attempt a sneak attack on the Black Fairy. Zelena catches the fairy and flings her into a wall before blasting her with magic, however, the Black Fairy is unaffected and stands up as she laughs at Zelena 's magic, walking up to her and redirecting her magic into the surrounding fairy crystals, imbuing them with her dark magic as Gideon and Regina arrive. As Zelena questions why the Black Fairy didn't use her own dark magic, the Black Fairy explains that Zelena's magic is at a disbalance due to her own lack of love and desire to prove herself while the Black Fairy 's own is stable and therefore useless to her since she is confident in herself. She then releases a powerful blast from the crystals and teleports Zelena and Regina out of the mines.

Regina gives one of the dark crystals to her sister, telling her to return to Oz. Taking her infant daughter Robin with her, Zelena summons a cyclone to go back but instead takes the Crimson Heart from Oz, believing it can revert the dark crystals to light if the heart absorbs all her magic. Zelena uses the heart to drain all the magic out of herself. Once Zelena loses her magic, the crystals change back to light magic. Emma succeeds in using a light fairy crystal to revive a comatose Mother Superior and restart her heart. ("Where Bluebirds Fly")