Emerald is a rare and valuable gemstone that is a variety of the mineral Beryl. Emeralds receive their beautiful green coloration from trace amounts of Chromium. Emeralds tend to have numerous inclusions, so unlike diamonds, which are magnified 10x to grade their clarity, Emeralds are graded by the eye. If "by eye" an emerald has no visible inclusions, it's considered flawless.
Emerald is the traditional birthstone for the month of May.