Diamond dust

Diamond dust is a feature introduced in Pokémon Diamond and Pearl. It was included in those games and later games as an Easter egg, though it has no known effect on gameplay.

History
On January 10, 2008, director Junichi Masuda announced on his blog that there would be a surprise for fans who played the games on January 12, promising them that something would be different. The date January 12 was chosen because it is Junichi Masuda's birthday. Fans found out that the surprise was that diamond dust fell in Snowpoint City instead of the usual snow. Diamond dust is also observed in that location on Leap Day, February 29.

Later games
The player can observe diamond dust in the Kalos region's Frost Cavern in Pokémon X and Y, as well as the Alola region's Mount Lanakila in Pokémon Sun and Moon and Pokémon Ultra Sun and Ultra Moon by playing on his or her birthday (subject to the birthday settings on the 3DS).