Destiny Crystal is one of the currencies.
How to Obtain
Crystals can only be obtained by acquiring more than 4 copies of a 4★ Magical Girl.
Each extra copy can be traded for 1 Destiny Crystal through the Magical Girl Menu.
Note: Extra copies of Akemi Homura (Megane ver.) can NOT be exchanged for Destiny Crystals, and she is not available for purchase in the Destiny Crystal Shop.