Emerald is the green variety of the mineral beryl, the same species that produces aquamarine, morganite, and heliodor. Its colour comes from trace amounts of chromium, and sometimes vanadium, within the crystal structure, an entirely different chromophore from the iron that colours aquamarine.
Unlike most other beryls, which form in granite pegmatites, emerald usually forms in mica schist or, in Colombia's case, in unusual calcite-bearing black shale. This happens because beryllium and chromium rarely occur in the same rock environment, so emerald deposits form under a specific, uncommon set of geological conditions. This is also why emerald crystals are almost always heavily included, a feature gemologists call the "jardin," French for garden, in contrast to the cleaner crystals typical of aquamarine.
Four localities produce most of the world's fine emerald. Colombia remains the benchmark source, its Muzo and Chivor mines known for a vivid green with a slight blue undertone. Zambia's Kagem mine is now one of the largest producers, typically yielding a more bluish-green stone in larger, cleaner crystals. Afghanistan's Panjshir Valley has produced rich green emerald for centuries, mined under some of the most difficult conditions of any gem locality in the world. Pakistan's Swat Valley, discovered in 1958, has since become known for material that rivals Colombian quality.
Because inclusions are almost universal in natural emerald, the overwhelming majority on the market, regardless of origin, are clarity enhanced with oil or resin to fill surface-reaching fractures and improve apparent transparency. This is a long-standing, disclosed industry practice rather than a defect, but it means an untreated emerald is genuinely rare.
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