THE EMERALD IS THE GEMSTONE OF MERCURY:

THE NATURE OF MERCURY
Mercury indicates education, letters, disputation, reasoning, brotherhood, interpretation, embassies, number, accounts, geometry, markets, youth, games, theft, association, communication, service, gain, discoveries, obedience, sport, wrestling, declamation, certification, supervision. Also, it is about weighing and measuring, the testing of coinage, hearing, versatility.
V. Valens
SYMPTOMS OF POORLY PLACED MERCURY IN THE CHART
When Mercury is afflicted or poorly placed in the astrological chart, the native will have communication, speech, learning problems. They will also have issues with reasoning, interpretation, business difficulties.
THE BENEFITS OF THE MERCURY GEMSTONE
Better communication, oratory, mental and intellectual skills, better mental reasoning and processing as well as improved business gain and growth opportunities.