To clarify: I didn't say anything further about the Rare Metal, because I wasn't sure if it was even the right ingredient for the crest.
And sorry about the Magical Mirror. I forgot that I have two of them.
Golden Bough + Secret Seed = The Theory of Evolution
The Theory of Evolution + Common Metal = Magical Mirror
Golden Bough location: Shiny spots in Glacier Road
Golden Bough drops: Elder Treant (in Fodra.)
The Elder Treant also drops the Secret Seed if you don't already have one.