Destiny Crystal is one of the currencies.
Crystals can be obtained by obtaining more than 4 copies of 4 ★ Magical Girls.
Each extra copy can be traded for 1 Destiny Crystal through the Magical Girl Menu.
There is a Destiny Crystal Shop where you can exchange 5 Destiny Crystals for a 4 ★ Magical Girl. This shop includes all unlimited 4 ★ Magical Girls. During Limited Gacha Events, the target limited Magical Girl will also be available.
Note: Extra copies of Akemi Homura (Megane ver.) can NOT be exchanged for Destiny Crystals.