We love the Paloma Picasso sugar stack rings, which are gold bands each with a single semi-precious stone. They come in citrine, rubellite, amethyst, blue topaz and several other stones. Pictured left is the large citrine sugar stack ring, which is available at Tiffany.com
for $1,575. The other rings are similar in price, so you can stack away so long as your credit limit holds out. The rings come in small or large, and are 18K. They really are lovely, with a nice minimalist sensibility.
You can get the same look on a budget with these stack rings from Ariella, shown to the right. The entire stack is $65 from Nordstrom.com. They aren't Paloma Picasso originals, and they are only brass coated in 18K gold, but if you want the look and Paloma's designs are currently out of reach, this will tide you over in the meantime.