That sounds about like Rainbow Crystals. You get them from discharging non-fodder units of gold+ rarity, with the amount gained being as follow:
Gold: 20 (non-event), 1 (event)
Platinum: 100 (non-event), 3 (event)
Sapphire: 1
Black: 400 (non-event), 15 (event)
So far there have only been 3 units to spend them on. Two blacks, each costing 1000 rainbow crystals, and one platinum costing 400 rainbow crystals.