If you walk into a dust cloud, you'll either find a Gem, Evolutionary Stone, or encounter a rare Pokémon. Gems can only be found this way in Black and White, though some gems can be found on the ground in their sequels. They can also help out your Pokémon in battles by powering up their moves with the right gem.
List of Gems
|#529||Wellspring Cave, Chargestone Cave, Mistralton Cave, Twist Mountain|
|#530||Excadrill||Challenger's Cave, Victory Road, Giant Chasm|