What's it called? It's a horn on Ralts and Kirlia, but if I keep calling it her "Chest Horn" I think she's gonna bonk me...

Stuck in the past. Now with internet!
I'm 16. Keep the weird stuff away from me.
(Mod is @Nava and an Adult)
What's it called? It's a horn on Ralts and Kirlia, but if I keep calling it her "Chest Horn" I think she's gonna bonk me...
i've always thought of it as a gem representing magic/psychic powers like on the lake trio/golduck/staryu/etc since they tend to all be red but that's a good question