Damage Crystals, are very rare Crystals and can be identified by its purple color and pointed tips. They're not useful though. When you are in their proximity, they do damage to your Health, rather similar to the Pain Fields found in Spawn Rooms.
If you want to find one, there are at least 2 in Hunhau. One is located in the North-central area of grid F-10 by a ramp and can be walked away from. Another one is located in the top left corner of cell J-14 in a hole by a ramp. Note that unless you are a VS MAX you will die since there is no way out of the hole. Another one, located in Adlivun cavern, is located at the upper, center of grid H-15. Just before you get on the UP zipline, look down to your left, into the crevice for it.