The nature inheritance mechanics differ from most games. According to Bulbapedia:
- In Pokémon Emerald, if the mother (in a male-female pair) or Ditto (in a pair that includes Ditto) is holding an Everstone, the baby has a 50% chance of inheriting that Pokémon's Nature.
- In Diamond, Pearl, and Platinum, inheriting Natures remains the same as in Emerald, except that passing on Natures via an Everstone will not work if the Pokémon are from different real-world countries (e.g. Argentina and Germany).
- In HeartGold and SoulSilver, whichever Pokémon is holding an Everstone will have a chance of passing down its Nature, regardless of gender, or whether it is breeding with a Ditto.
- In Black and White, inheriting Natures remains the same as in HeartGold and SoulSilver, except that passing on Natures via an Everstone will now work even if the Pokémon are from different real-world countries.
From Black 2 and White 2 onward, if one parent holds an Everstone, its Nature will always be passed down. If both parents are holding an Everstone, a parent's Nature will still always be passed down, but which parent's Nature is passed down is randomly determined.
Hope I helped. :)