Dr. G. S. Kapoor. (1994). Gems and Astrology. Ranjan Publications, New Delhi. p. 93-95:

• "Emerald is the gem stone ruled by Mercury. Emerald will prove very beneficial to those in whose nativities Mercury is the lord of auspicious house.

• If such a Mercury is badly disposed or is afflicted by conjunction with or aspect by malefics Emerald will neutralise the evil effect of such adverse influences. 

• Rituals:

  • Emerald should, as far as possible, be set in a silver ring on a Wednesday.

  • Fridays and Saturdays will also not be inauspicious for this work.

  • Then after observing the prescribed rituals and reciting the following Mantra 108 times, the ring should be worn in the little finger of the right hand about two hours after Sunrise, on a Wednesday of the bright half of the lunar month. 

• Aum bum bhudaya namah

• The weight of Emerald should not be less than 3 carats. Emerald is an expensive stone. Those who cannot afford to buy an Emerald may use any of these substitutes — Aquamarine, Peridot, green zircon, green Agate or Jade; but the Jade which is sold in the market for Rs 1 or Rs 2 a piece is of no use. 

• Important: Emerald and its substitutes should Important: Emerald and its substitutes should never be worn with Red Coral, Pearl and Yellow Sapphire and their substitutes."

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  For all remedial gemstones, be sure to cleanse the stone energetically with extreme attention to proper mantra.

  Flawless gems are expensive and hard to find. Use only a trusted Vedic-trained gemologist. It might be easiest to start with single gem and combine them as your finances allow.

  According to Dr. G.S. Kapoor (1994, p 91)

  • " It is very important for all gem stones that before wearing them they should be kept immersed for sometime in unboiled milk or Ganges water.

  • Afterwards the stones should be worshipped with flowers and incense, and the Mantras of the planets should be recited 108 times."

[BPL note on Pearl + Panna

I take respectful exception to Dr. Kapoor's view that Chandra-ratna should not be mixed with ratna for the cool graha. IMO Chandra is a "bridge" graha who is friendly with both the "hot" group (Guru, Surya, Mangala) and the "cool" group (Budha, Shani, Shukra).

Moon has no enemies therefore it is probably OK to wear Pearl with Emerald, when these stones are both flawless and properly set. It may be preferable to wear Budha-ratna on the left hand and Chandra-ratna in the right hand, or to wear the Pearl as a pendant to touch the 4th chakra, if both stones are to be worn in the same period of life.] 

Naturally for such a prescription to be feasible, Chandra and Budha must occupy mutual trines and both should rule good houses from lagna, e.g. for Thula lagna Chandra = L-10 and Budha = L-9.