These didn't turn out as nicely as I hoped, but maybe I'll try again in the future and see if I can get a better result. I grabbed out some polished stones for inspiration, and for this I was trying to emulate a banded amethyst.
I started with Sinful Colors Enchanted, which is a nice dark purple. I then sponged on highlights using Color Club Lavendarling, a warm mid-tone purple.
Next, I added the bands and striations using a small brush and Essie To Buy or Not to Buy. It's a very pale lilac that almost appears white on top of all the dark purple.
Lastly, I added rough 'tips' in NYC Pinstripe White to reflect the white edge to my amethyst stone. I didn't make them perfect french tips; rather, I wanted them a little lumpy and organic.
Trying to pose fingers with a rock is rather challenging!