is simply a variety of the mineral beryl. Primarily green, emeralds often display tints of yellow and blue. Too much blue, however, and a beryl is classified as an aquamarine. Not green enough, and it's known as a green beryl. That's why the greener an emerald, the more valuable (and eye-pleasing) it is. With color and clarity being their defining features, the most desired emeralds come in shades of lustrous green and with as few inclusions as possible (although rare are emeralds that are completely "clean," or inclusion-free).
High-quality emeralds come from the mines of Zambia, Brazil, Pakistan, Zimbabwe and Russia. Interestingly, emeralds' color and inclusions betray their country of origin. Most of the world's finest emeralds are still mined in the jungles of Colombia. Already in the sixteenth century, the Spanish Conquistadors, grasping for gold, were stupefied to find, in the lands now constituting Colombia, massive loads of beautiful emeralds unrivalled by any they had ever seen. The natives had mined emeralds there a millennium before the arrival of the Europeans, and the Inca and the Aztec peoples liberally used emeralds in their symbols and decorations.
To this day, people who believe in the transcendental powers of crystals attribute to emeralds the virtue of increasing cleverness and preserving love. And all can agree with the first-century Roman historian Pliny the Elder that "nothing greens greener than emeralds."