I tried to emulate amethyst geodes for this design. Every time I look I can't decide if I succeeded or not. They still look neat, so it all works in the end?
I started with a base of OPI Thanks a Windmillion, which is a dusty neutral green. It reminded me the most of the rock usually associated with amethyst geodes. Next, I painted on loose shapes in Zoya Payton on each nail. I considered painting on the inner facets all by hand, but realized Payton was the perfect amethyst color, and it has a great depth to it with tons of holo sparkle.
Next, I painted on the side facing crystals with OPI Alpine Snow and a small brush. I highlighted the tips with a touch of Color Club Sweet Pea. I also added a touch of an outline in Sinful Colors Coffee. As a last detail, I used Salon Perfect Loves Me Loves Me Not to give the outer stone some texture. It's a tiny matte glitter speckle polish with pale pink/sand and black microglitter.