What do you with a odd keystone in Pokemon diamond?

In the game Pokemon Diamond, the Odd Keystone is used to unlock special events. When given to the hiker in Veilstone City's Pokemon Center, the Odd Keystone unlocks the Celestic Town Ruins. To access the Celestic Town Ruins, the player needs to travel to Celestic Town and take the northern exit. This will lead them to a small field, where they can find a hiker in the middle of the field. If the player talks to the hiker and presents him with the Odd Keystone, he will open up the gate leading to the ruins.