Emerald derives it’s glorious green color from the presence of chromium and vanadium. Emeralds are rarely flawless, therefore stones are often enhanced by oiling them to hide the imperfects. As well, the step cut (or emerald cut) is used to maximize the stone.
Ancient mythology teaches us that the emerald was associated with the Mercury, who was known to be the messengers of the gods, and of travel. It thus makes perfect sense that the emerald is commonly referred to as the Travelers Protection Stone.