Like any other knowledge, astrology also has two components a) Theory b) application or technology. The purpose of astrology is not just to give us a broader knowledge of why and how planets and stars influence different aspects of our daily life, but also how to harmonize these influences through different remedial measures like gem therapy, yoga , meditation and mantra.
Gem therapy or use of gemstones is one of the most powerful and effective remedy to improve the planetary effects in our life. It recommends both lifetime gemstones based on our natal chart and temporal gemstones based on present dasha or transit conditions with respect to our natal chart.
A gemstone generally strengthens the influence and effects of a particular planet whether they are positive effects or negative effects in our life as per our natal chart.
The basic principle of gem therapy is “ a planet should generally be strengthened if it is a good planet but week and is not effective enough.. Conversely a negative planet should never be strengthened because it is debilitated , and a malefic planet located in difficult houses like 8th house etc.
The most important and difficult task is to decide on which planet to strengthen based on the status of planets both naturally and temporally , as per the rules of malefic and benefic planets and house rulership.We also have to consider the Dasha system , the main period, sub-periods that the person is experiencing at a given time. One also has to decide the strength and size of gemstone, for it to be effective. These are complex rules and a competent astrologer must consider all factors before he/she recommends the same to a person.
As per Vedic Astrology, the main Gem stone for Rahu is a hessonite garnet ( Gomedha) . There are two main varieties, one is Cinnamon in color, and the other is golden. Other golden or cinnamon colored grossularite garnets can be used as cheaper substitutes. The weight should be at least 3 carats ( 5 Rattis) set in gold or silver and worn on the middle finger of the right or left hand. Darker red or opaque garnets should not be used as these increase the negative ( eclipse – causing ) power of rahu.
Such gems should be put on first on a Saturday or the day of the planet that rules the sign where it is located in the natal chart. Rahu should be in favourable signs or those favourable to the planet that rules it. Alternately one can choose a good Muhurat for healing on that day.
Hessonite Gemstone for Rahu is relatively safe (except for the darker ones). It must not be worn , if rahu is debilitated in the natal chart that is Rahu is located in Scorpio in the birth chart. Rahu has predominant influence in the materialistic age (the force of Maya) and brighter gems for it help remove the darkening effect upon us. Hessonite can give a boost to one’s career, finance etc especially when a person is experiencing Rahu period /sub-periods etc. However it should be avoided in case it is in a trine or afflicting benefics like Jupiter in the natal chart. This will then neutralize some of the positive results of Jupiter/Moon etc.
Reference : Astrology of the Seers by David Frawley.