Selenite and Satin Spar are both members of the Gypsum family. Their energies are similar however, selenite is known to be a stronger energy. To help you best identify - Selenite is typically clearer whereas Satin Spar is typically a cloudier/milky color and texturally appears to have a flaky threaded texture. Satin Spar is often mis-labeled as Selenite however, given the similarities in energetic signal you may not truly notice any difference in your work with one vs the other.