13 crystal skulls set in the cardinal directions North, East, South and West. These crystals were set on a trail in the Santa Cruz mountains and were set up to activate the leylines and crystalline consciousness. You can bring crystal skulls and a center point and install this grid anywhere, just go clockwise placing your finger over each skull and om, then when you finish the circle finish with OMing over the center crystal to fully activate it. Begin with the crystal skull which is closest to you (usually start with East since it is the best energetically for crystals if you know its exact location).
A crystal skull grid which uses 13 crystal skulls is specific for the planet, other configurations do other things and serve different functions.