Sandgem Town is the second town in the Sinnoh Region that players encounter during their travels in Pokémon Diamond, Pearl and Platinum. It is the first town in Sinnoh with a Pokémon Center and a Pokémart. This is where your second rival lives, which is either Dawn if you chose the male playable character, or Lucas if you chose the female. Its key features include the beach in the south and Rowan's Lab.
Locations of Swarm Pokémon can be discovered here. Go to Dawn/Lucas's house and talk to his/her sister and she will tell you that a Pokémon has been found on a certain route (for example, Beldum on Route 228.) Going to that route will allow you to find that Pokémon, which is normally unobtainable there.