Altar tiles, offering plates or altar pentacles are old school magick.
In ceremonial magick the four elements are represented by for important elemental ritual tools. The wand for fire, the dagger for air, the chalice for water and the disk/plate/tile/pentacle for earth.
Energy raised during ritual is typically put "into" something and discharged. The altar tile, representing earth, is perfect for manifesting energy on the earthly plane. Food or other items are placed on the charging plate and the ritual energy is visualized being transferred into the items on the tile/plate via the wand or dagger.
Selenite is very sacred to the moon and all Luna goddesses.