The birthstone for the month of May is the emerald, a green gemstone that is the green variety of the mineral beryl. Emeralds range in color from slightly yellowish green, to pure green, to slightly bluish green, with pure green emeralds being the most popular and valuable.[1]
Emeralds are mined in Colombia, and the May birthstone is often treated to improve its color or clarity. Common treatment methods include dyeing, where paler emeralds with multiple fractures may be dyed green to enhance their color. [1]
In addition to being the birthstone for May, the emerald is also said to symbolize love and rebirth.

Sources:
1. Geology. "Birthstones by Month: Chart and Photos. <https://geology.com/gemstones/birthstones/>