The gemstone Emerald is the green variety of the mineral beryl. Emerald is the May birthstone.
The wonderful green color of emerald is unparalleled in the gem kingdom. Emerald’s precious green color is caused by small amounts of chromium and enhanced by traces of iron. Unlike other beryls, emeralds usually contain inclusions and other flaws. These flaws are not looked on as negative aspects for emerald like they would be for other gemstones. Indeed, these flaws are considered part of the character of the stone and are used to assure the purchaser of a natural stone.
Even artificial emeralds often contain flaws, however, as the process of growing artificial emeralds mimics the way nature does it (slow crystal growth from a molten mix).
Nearly all emeralds, even many “natural” stones, have been treated to improve clarity, generally by immersing them in oil. For gemstones, a green colored oil is sometimes used, “improving” the color as well. Unfortunately, this oil may evaporate over the years, making flaws appear where none were visible at the time of purchase. A high-grade mineral oil may be used to improve the appearance again.
For more info on Emeralds, click on: https://www.gia.edu/emerald