Valued especially for their color, emeralds and aquamarines are two gem forms of the mineral beryl (beryllium aluminum silicate). Pure beryl is colorless and rare. Impurity atoms incorporated as the crystals grow tint them a range of colors. Emeralds get their rich green color from traces of chromium or vanadium...or both. Emerald is the May birthstone. Aquamarines come in shades of blue and blue-green, depending on the amount and chemical state of iron impurities locked inside. Aquamarine is the March birthstone.