The Odd Keystone is an item you obtain by Diving. To Dive you go to the Lab in the middle of the lake in Lagoona Lake. Talk to a lady and she will let you dive in the water with a submarine. NOTE:You only have one free chance to dive, after you must purchase batteries from her for Robux to Dive again. Once in the water find the light shining and press that. Look for the Odd Keystone as you search the wall for items that lay there. Carefully dig piece by piece so the whole wall does not collapse. Once you find it press it and come back up to the surface, and you will get it.
To use it: Goto Route 9, either from Lagoona Lake, or from Rosecove City. Close to the end of Route 9, there is a strange well, which when the odd keystone is thrown in, the pokemon Spiritomb is found - this pokemon has genders.