Abstract: Emeralds are cherished and coveted by gem dealers, collectors and jewelry designers, owing to the unique and mesmerizing colour. As international emerald market booming, there exists strong demand for a scientific, standardized, universal colour grading system of emerald. Based on research on emerald of decades and Munsell colour theory, combined with trade demand, Guild Gem Laboratories developed a scientific colour grading system of emerald, with colour grade including Light, Medium, Intense, Vivid and Deep. Guild also introduces the Verdant Green term representing the top colour of emerald to the market, aiming to promote the emerald trade rapidly, healthily and orderly.