Emeralds are green colored Beryls. They are essentially the same stone as Aquamarine. Some other stones from Beryl family are Red Emerald, Heliodor and Goshenite. In Greek, emerald is smaragdos, but in Middle English the stone was called esmeralde.
Egyptians associated the stone with the rebirth and fertility due to its green color. It was thought to help in childbirth and improve fertility. Most natural emeralds have some flaw in them because they grow in other minerals. Emerald is also a birth stone of May.