Alexandrite is a highly prized variety of chrysoberyl. Alexandrite only forms when aluminum/beryllium and chrome (which rarely occurs in nature) come together, accounting for its rarity. On Mohs’ scale of hardness, alexandrite is 8.5. It has a strong vitreous luster and primary sources include Sri Lanka, Zimbabwe, and Brazil (Minas Gerais).
Color
Alexandrite is typically found in blue, blue-green, and brownish-green shades. However, this phenomenal gem is known as "emerald by day, ruby by night" because of its ability to change color, depending on the type of light in which it's viewed. In the daylight and fluorescent light, alexandrite appears in shades of blue and green, but in tungsten (incandescent) light, it appears in shades of red. Other alexandrite color changes are possible (from blue to purple, from greenish blue to reddish blue, from brownish-green to brownish-red, etc.) and also highly prized.
   
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