The Dawn Stone is a 4th Generation evolution stone.
A most curious thing of it is that Pokèmon will only evolve with it if of a determinate gender: by using it, a male Kirlia can evolve into Gallade and a female Snorunt can be evolved into Froslass.
Only two can be found during the adventure. Though, more can be obtained by having a high-level Pokèmon use Pickup
It can also be obtained by getting 3000 Pokeathlon points in HeartGold/SoulSilver in the pokeathlon after you get the National Pokedex.
In Pokemon Black and White it is found on the way to victory road, before you come to the gates.