What is the enigma stone in heartgold?

There is no "enigma stone" in Pokémon HeartGold. There is, however, the Enigma Berry, which is a rare berry found in the wild.

It's possible you're thinking of something else. Could you clarify what you're looking for?

Perhaps you're thinking of:

* The Enigma Stone in the Pokémon Adventures Manga, which is a mysterious stone that grants the holder the power to control time.

* The Enigma Orb from Pokémon Mystery Dungeon, which is a powerful artifact that grants the player the ability to communicate with other Pokémon.

Let me know if you can provide more context, and I can help you find the answer you're looking for!