Alexandrite is known as "Emerald by day, ruby by night" because it has one of the most dramatic color changes among gemstones, such as green in sunlight and red in incandescent light. The very precious habit of alexandrite color change is caused by the presence of minor impurities in the element chromium. The most desirable Alexandrite colors are those that have a very clear color change.
garnets are a category of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. all species of garnets possess similar physical properties and crystal forms, but differ in chemical composition.