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.
The feature was revealed on Junichi Masuda's official blog on January 10, 2008.
On January 12 of every year, diamond dust falls in Snowpoint City instead of the usual snow. Diamond dust is also observed in that location on Leap Day, February 29.
The player character can observe diamond dust in the Kalos region's Frost Cavern in Pokémon X and Pokémon Y, as well as the Alola region's Mount Lanakila in Pokémon Sun and Pokémon Moon and Pokémon Ultra Sun and Pokémon Ultra Moon by playing on his or her birthday (subject to the birthday settings on the 3DS).