Self-healed crystals have been broken off or damaged at some time butn they begin to grow again after that damage occurs. It may take thousands of years for the crystal to heal but it does so quite successfully and continues to grow larger.
The damage is caused, most often, from tectonic activity or rapid changes of temperature that cause the breakage of the crystal.
Crystals will detach from the base or bed they are growing from and end up on the floor of the place they are growing or on a shelf or ledge. The base is left without the base touching anything since it will generally land on its side. The crystal then remains in the growing environment and the mineral rich fluids and vapors present cause it to continue growing for centuries.