Color-change is most popularly noted to be a defining trait of the gemstone known as alexandrite. This stone is a variety of the chrysoberyl species, and remains to be the most expensive variant per equal quality characteristics. Alexandrite in its finest forms, can change from ruby-red to emerald-green depending on whether you view it under incandescent light (candle-light) or normal daylight. Top qualities can command tens of thousands of dollars, although synthetic variants (man-made versions) also do exist.
Next up, we have a variety of diamond that not many people are familiar with. These color-changing rarities have been termed as "chameleon diamonds" and change their color due to a combination of light and heat. Many are a subtle green, but can change into a vibrant yellow or brownish yellow when subjected to the said conditions. These diamonds are quite rare, and not very much is documented about their nature. Still, it's an interesting quirk to have, especially for the world's most famous gemstone.
Thirdly, we have color-change sapphire. Sapphire as you might already know, does not only come in the vivid blue colors that we usually associate them with. They belong to the mineral species known as corundum (along with their cousin- the ruby). While blue sapphire is colored by a charge transfer between trace atoms of iron and titanium, the presence of other elements can also induce a wider variety of colors in the mineral species. The presence of certain elements like chromium and vanadium have been associated with the color-change phenomenon seen in natural sapphires. Synthetic or man-made corundum is usually doped with different amounts of vanadium to create artificially grown color-change sapphires that can shift from a reddish purple at best, to a violetish blue in opposite lightings.
Be aware that aside from synthetic counterparts, glass can also be manufactured with trace elements that allow for color-change. This makes it possible to create imitations of these phenomenal stones, so have your gemologist take a look at your gems, to give an better opinion on the validity of their identities.
