An emerald is a form of which mineral?

Emerald is one of the gem varieties of the mineral beryl. They are highly valued stones used for jewelry. Gems generally get their color because of certain trace metals or impurities contained in the mineral, and in the case of emeralds, they contain traces of chromium, or sometimes vanadium, giving them an intense green color. Gems of lighter green color (and sometimes even yellowish color) are referred to as green beryl.

Source: Geoscience Australia