Some wore shiny rocks on their heads. I know on their heads is preposition but confused if it's adjective phrase because it describes rocks or adverb phrase because it answers where rocks were worn.

Question

She Wins the Race, Morning Girl's mother, had said a new sister would add her smiles to the family

Morning Girl had awaited her baby sister with great curiosity. Again sister is noun does it make it an adjective phrase or describes how awaited making it adverb phrase

Answer 1

"Adverbs can be single words, or they can be phrases or clauses. Adverbs answer one of these four questions: How? When? Where? and Why?"
http://www.chompchomp.com/terms/adverb.htm

Adjectives describe nouns, for the most part.

So ask yourself, "What question is 'on their heads' answering?"

Let us know what you come up with.