Signs of weakness:
Signs of a weak Mercury are lack of intelligence, poor communication skills, speech defects, weak memory, inability to calculate, immaturity, childishness, foolishness, lack of self control, addictions, and dependencies. The individual may be slow, dull, daydreaming, or irrational. The mind may be out of control.
Physical factors are weakness of the nervous system, nerve pain, tremors, anxiety, insomnia, palpitations, dry or itchy skin, hypersensitivity of the senses, nervous indigestion, and allergies. The lungs and heart may be weak. The individual will easily suffer from pain and anxiety.
Mercury is weakened by aspects from malefics (like Saturn, Rahu, Ketu, the Sun, and Mars), by being in its fall (Pisces), and by residency in difficult houses (the sixth, eight and twelfth).
Mercury is a generally benefic planet for Taurus, Gemini, Virgo, Libra, Capricorn, and Aquarius ascendants. For such charts, we can apply the remedies for Mercury to increase the higher Mercury qualities like discrimination, intelligence and power of communication.
The main gem for Mercury is an Emreld Gemstone. It should be at least one and half carats in size, set in gold, and worn on the middle finger of the right or left hand. Sometimes it is recommended to be worn on the little finger, as it is the finger of Mercury.
Substitutes are peridot, jade, green zircon or green tourmaline. Such Gems can also be worn as pendants or strands, hanging down to the region of the neck. As rings, they should be at least three carats where as in pendants it should be atleast five carat.
They should be put on first on a Wednesday, when the Moon is waxing, and preferably when Mercury is strong, in friendly signs, its own sign or exalted.
When not to wear a gem for Mercury:
Gems for Mercury are relatively safe, harmonious and balanced. They have the Mutable nature of Mercury, however, and so we should be careful wearing gems for Mercury with those of malefics like Saturn, Mars or the Sun, as they may amplify their malefic influences. We must be careful what influences these gems may pick up, or may have had when we first got them.
Astrological factors are Mercury as a malefic lord, particularly for Aries or Scorpio ascendant, especially when aligned with Saturn or Rahu.