It's likely you're thinking of the Oval Stone, which is an item in Pokémon Diamond, Pearl, and Platinum. This item is used to breed Pokémon, increasing the chance of getting a Shiny Pokémon egg.
SoulSilver does not have the Oval Stone item.
However, you can still breed Pokémon in SoulSilver to get Shiny Pokémon. The chance of getting a Shiny Pokémon is 1/1365.33 without any special items or methods.