The basic principal of gem therapy is simple. If a planet is weak and is also beneficial and auspicious in the chart, then such a planet should be strengthened by gems of that planet.
Each astrological planet is associated with a particular gem(s). The Bhashma of Gems (powder extract) is extensively used in Ayurveda for its medicinal properties. For astrological purposes, the gem is embedded in the ring or pendant in such a manner that the bottom is open, allowing the gemstone to touch the body (Diamond being an exception). Varahmihir, the author of ancient astrological text, Brihat Samhita, suggests that a gem having a good luster, pure and clear-color and without flaws ensures good luck and protection.
There are two theories on the use of gemstone for astrological purpose. The first is based on the principle of enhancing a particular ray or color effect where the gemstone acts like an amplifier. The origin of this idea can be traced back to the Rig-Veda, where humans are advised to use the seven gems to gather the rays of the Sun onto our body. The second theory envisages that the gemstone acts like cosmic filter, preventing harmful rays from affecting our bodies. Diseases are caused in the body due to deficiencies and the use of certain gemstone supplement the deficiencies, which results in improvement of health, mood and life force.
Selection of a particular gemstone for vedic astrological remedy is a complex process and only a competent astrologer is capable of making the right choice. Inappropriate use of gemstone is known to have caused harmful side effects. Gem should be of good qualities and without flaws.