The medium- to dark-green gem variety of the mineral beryl, Be₃Al₂Si₆O₁₈, crystallizing in the hexagonal system. A flawless emerald with good color is one of the most sought after and highly prized of all precious gems. Emerald is restricted in its occurrence, and seldom are exceptional stones found; most emeralds are flawed and cloudy, and few stones command high prices. What constitutes a true emerald is well defined by the gem dealers, and the dividing line between emerald and mere green beryl is sharp.