Hello! You are looking for "phenomenal" stones, and they are honestly the most exciting part of a collection. I am personally obsessed with the "horsetail" effect found in Russian demantoids from Tsarina Jewels https://tsarinajewels.com/ . It is a unique inclusion that looks like a spray of golden fibers and it is incredibly rare. Then there is the color-change in alexandrite, which is like having two different rings in one. When you explore gemstones and high end jewelry, these optical effects are what really drive up the value and the "wow" factor. I also love the "fire" in certain garnets that actually creates more rainbow flashes than a diamond does. My advice is to look for a stone that has a personality of its own. It becomes a great conversation starter because people can't stop staring at how it reacts to the light.