Called TV or Window Selenite. This is different than they satin spar sticks you might be more familiar with. It appears opaque but then when placed on top of printed material, it projects the image to the surface of the stone.
It is also the perfect tool to use in order to cleanse your other crystals.
Selenite itself does not need to be cleansed, but naturally discharges incorrect energies. By laying it down flat and placing your stones on top of the plate, the selenite will act as a purifier and will help to remove accumulated energy.
Size: approx 60-75mm x 3/4" thick