When discussing gemstones, most people think of things like diamonds, or rubies. But the definition of gemstone is any substance that is cut and polished for use in jewelry. That being said, my personal definition of gemstone is anything that make your eyes sparkle. Diamonds, sure- but also opals, petrified wood, abalone, and pearls.
I adore gems and minerals of all types- and as long as the material is durable enough to stand up to being worn, I consider it fair game to design a piece around!