Diamond dust is a special variation of snow that appears in Snowpoint City, at Mt. Silver's summit, and in Icirrus City on certain days. It has no effect in battle.
Director Junichi Masuda announced January 10, 2008 on his blog that fans should play Pokémon Diamond and Pearl on January 12, his birthday, for a surprise. The surprise was revealed that Snowpoint City has sparkling diamond dust instead of the normal snow that floats down. It is purely aesthetic, since it only occurs within the city where no battles are possible. It has been determined that other special weather conditions such as thunderstorms happen on predetermined days throughout the year, such as:
July 11, December 2 and December 31 (Rain in Route 213)
October 30 (Diamond dust in Snowpoint City)
Diamond dust is also available in Pokémon Platinum, HeartGold and SoulSilver, and Black and White.