Gems are very powerful, and wearing the correct gem can bring great benefits. Wearing the wrong gem can cause great harm to the individual by strengthening malefic (bad) planets.
A Vedic Gemstone prescription is performed by a professional Vedic astrologer called a jyotisha who make a diagnosis concerning the correct gems for you based on your natal chart.
The accuracy of this information is critical. I have seen gems bring amazing benefits when properly prescribed, astrologically constructed, and charged by mantra.
There are two methods of Gem selection:
1. Pratikul--A Pratikul Planet's gem is chosen using a method for overcoming a weak planet in a person's chart based on the concept that a 'weak' (inauspicious) planet is strengthened by wearing a gem to enhance that planet and change the planet from being inauspicious (weak) to make the planet auspicious (strong).
2. Anukul--An Anukul Planet's gem is chosen using a method for strengthening an auspicious planet(s) (exalted, mulatrikona, own sign, benefic, great friend, etc.) in a person's chart based on the concept that a strong or benefic planet will bring good results or luck to the individual wearing that gem. This method is safer and generally preferred.
Also the Chart must be carefully analyzed to determine hidden and powerful energetic natural, temporal, and compound relationships that Grahas (planets) have with one another prior to recommending any gemstone for a client.
* Also it is important when combining Gemstones in a setting that the gems are properly combined so that there is no conflict between their various energies. These and other matters are to be carefully considered by the Vedic astrologer trained in such things. I have spent many years studying and observing these effects firsthand and also collaborating with other Vedic astrologers and getting feedback in these very critical matters.
I feel a Vedic Gemstone Prescription is valid, having seen the effects--both beneficial and detrimental--to many people over the years.
Vedic Astrology and Remedial Measures
How Gems are charged on Yantras with Prayers & Mantras
The Power of Vedic Astrology for your Life Path