A phenomena is a special encounter which occurs randomly and cyclic in all areas of Unova and has 4 appearances: Rustling Grass, Rippling Water, Flying Shadow and Dust Clouds. There are exclusive species to phenomena, encounters have a >50% chance to have at least one 31 IV, and it may drop Mysterious Gems or Fossils.¹   EDIT: Please see the second answer for updated calculation.   Original post with presumably incorrect assumptions: It is believed only a single pheno