>>57766777The Oval Stone, while present in the table of treasure items, and fully implemented with an underground sprite and functionality in the 'Treasures' bag menu, is programmed to never spawn under any circumstances. Not in Diamond, or in Pearl, or in Platinum with either even/odd trainer IDs, and not pre- nor post-National Dex.
This is really strange; sites like Bulbapedia and Serebii report that you can dig up an Oval Stone, while Psypokes and the Prima guidebook do not. Though, none of them give you the precise odds and just say things are "very rare".
