First and foremost, great job participating in the Jam! It's tough to come up with a story from a prompt, type it out, and submit it all within 48 hours. The idea to compensate around this by having a "story within a story" was a clever solution that worked pretty well in this case.
Echoing what small_grey_owl said, I think giving yourself the opportunity to brainstorm will help with finding a full story you want to write out. you should also ask yourself questions while you're writing - Where are the characters located - a village? A city? What is the story that Rose and Piper Mention? Is there anything special about the quartz? Why is Billy the most obnoxious elf you can meet?
Asking these sorts of questions while writing (or after writing), and adding to your writing based on the answers you come up with can help you develop your story and enrich the world you're creating. It's clear you have a lot of ideas on the world that are hinted in the writing, I'd love to see it further developed!
Thanks for sharing this for the Jam!