Emerald derives from the term 'smaragdos', meaning 'green stone'. Emerald is the most precious gemstone in the beryl crystal group as a result of its incomparable green, caused by the presence of chromium. Only the finest emeralds are transparent, with most featuring inclusions, often in three phases; of gas, liquid and solid. Historically the finest emeralds were found in Colomnia and India, where one Maharajah was weighed in emeralds. It is also the birthstone for the month of May.