Rough emerald. Close-up of uncut emerald crystals. Emerald is a variety of beryl, a beryllium aluminium cyclosilicate. Whereas beryl may vary in colour, emeralds always occur in hues of green, thanks to trace amounts of chromium in the composition. When pure and flawless, emerald is the most precious green gemstone known, the deeper the green the more desirable. This specimen was found in Muzo, Colombia, and is roughly the size of a walnut.