"Adult squirrels will often coat a regular acorn with a mucilaginous secretion that gives it a shiny, "golden" coat. This sparkly trinket is used to attract possible mates. Warning: do not eat."
The Golden Acorn is a Scavenger Hunt item located in the GPC and Wilderness in Psychonauts. It is held by a squirrel who will run into a tree if you get near it. If you use Invisibility, however, you can walk up to the squirrel and take the acorn without it noticing.