How do you get in to Lake Valor Pokemon diamond?

In Pokémon Diamond, you can gain access to Lake Valor by obtaining the Relic Fragment from Celestic Town and finding the blue stone in the entrance to Wayward Cave on Route 206 near the entrance of town.

When you approach the the blue stone, you will be prompted to insert something. Use the Relic Fragment on the blue stone to gain entry to Lake Valor.