Plasmic Crystal is a tier 5 ore found on Strange Sea and Atropus planets.
It can also be produced and easily automated even in early-mid game through extraction Bloody Rock or Blood Crystal - Both of them can be crafted in Liquid Mixer with liquids that are obtainable using Infinite mixing feature. It is extremely useful if you really need Plasmic Crystals and Quietus Ore (Which you can extract from Plasmic Crystal itself) and don't have protection against Atropus Maddening atmosphere.