Fortunite crystals are crystals with magical powers used for different purposes. Like telling the future or, like in Animal Crossing Agents, used to fuel the crown of nightmares, created by Evelyn's father, Cyrus and other scientists. (More about it in Animal Crossing agents below)

edit Animal Crossing If Madame Zoe moves into your town, she will open a fortune teller's shop and use fortunite to sell her fortunes.

edit Animal Crossing Agents SPOILER WARNING!!! Plot and/or ending details follow.

A fortunite crystal was discovered by Cyrus Boudreaux and it was used to fuel the Crown of Nightmares, but he took it to his mansion when he realized Vole Vinnie was evil. When destinite is affected by high frequency sounds (such as DJ Candy's towers), it turns into fortunite, which Brandi and Esma took advantage of. Cyrus owned the crystal until death, and in his will, he left the crystal and his mansion to Violet Nightshade. The fortunite also allows people to see the future, as Madame Zoe did to find out who would be named in her uncle's will. When she finds out it is not given to her in the future, she replaces the fortunite with a fake crystal and hypnotises Zombie Carl into smashing the fake crystal so everyone goes home and she can use the crystal freely, but her plot was discovered by you. When joined with the Crown of Nightmares, it opens a path to the Nightmare Realm. Vole Vinnie obtains the crown and the fortunite and tries to use it to take over the world, but Evelyn Gray sacrifices herself to save the world. Both items were transferred to a secret location for research. There is also a replica of the crystal in your Trophy Cabinet at the Headquarters.

SPOILER WARNING!!! Spoilers end here. 

edit TriviaIt is interesting to note that people purely came to the will reading for the crystal. Trevor wanted it as a prop for his play and Nicole wanted it to see new fashions.