There are two gemstones that change color.
In sunlight/natrual light they are green. In artifical light, they are red.
judging by what you said, I would guess something like a faceted quartz ball. As sunlight passes through it, it creates multiple rainbows and from different angles, each color is displayed.
I looked up some various crystals that I hadn't considered. I've decided to make Luna's stone from rainbow quartz and Moony will get what i intended at first...moonstone(but a molecularly aligned variety.)
Though I based the focusing crystals on lasing crystals, I may make the twin's stone have multiple cores joined around a common axis and surrounded by diamond. After having had a look, I decided to make their lightsabers look more like the Gundam beamsabers. They're not too far a stretch from the regular lightsabers, and that wider section would easily accommodate the compound crystals. Seeing as it's the twins, they'd also be competing to see how different they can make theirs, and several others will as well. In the next chapter you'll see four new variants.
Until next time...