Homebrew spell ideas. Help needed

I am playing with my sister and her friends for the first time and I have a cool idea for my arcane focus. I may have a spell idea that could possibly be tied to my arcane focus. Backstory for the focus. Mother is an elven wizard that teaches at a college and she gave this ancient gem to me(arcane trickster tiefling). Somehow or another the gem gets embedded under my skin in shards or whole(haven't worked that out yet). I'm wondering 1.) if I can use it as a focus like that 2.) are there any spells that have a scatter shot shard type affect. I have an idea for a spell if not and if I can use it like a once a day type spell. Either way I have ideas to play off of this.