Corrupted crystals are simply overaspected.
From this earlier thread:
Since the Calamity, there has been much imbalance in the elements. Founts of over-aspected aether erupt from the soil and manifest into corrupted crystals. These crystals, in turn, attract the most unsavory of creatures, the most common of all being elemental sprites who, upon absorbing the crystals, become corrupted themselves. The Brass Blades are seeking adventurers to comb the desert and find the crystals, slaying any corrupted sprites encountered.