Gems traditionally play an important part as a remedial measure to help strengthen the good aspects of a person's chart and ameliorate any unwanted or harmful influences in the chart. There's a whole art and science involved as this approach has been part of Hindu culture for well over a thousand years. A gem, of sufficient purity and carat, worn on the correct finger can indeed help alter a person's course through life in ways that conventional western science is still hard put to explain. However, just as for all of Jyotish, some forward-thinking modern researchers are beginning to find evidence and piece together a rigorous model for why the effects of simply wearing a gem might have such profound impact. Placebo effects and cultural conditioning aside, there is solid evidence that this style of therapy does just what it purports to do. You might think of gems acting much as a homeopathic remedy works—at a more subtle energetic level which is nonetheless real and has real effects.
The traditional theory asserts that stress in a a person's birth chart corresponds to stressful patterns, often between planets (subtle forces of nature). The planets all map to what are known as the elements (tattvas) in yoga and Hindu culture. The below picture depicts the relation between the five elements and five fingers. Hence, once a person identifies what help is being sought, an astrologer takes over and crafts a prescription for the appropriate remedy. The last step? Why, go to the jeweler, purchase the precious stone in its correct setting and then wear it as advised.
As an example, the most auspicious planet, Jupiter (Guru), is related to yellow topaz and yellow sapphire. If a person wanted to strengthen or harmonize this signifier of good fortune, wisdom, success and health, the appropriate formulation would depend upon where Jupiter was placed in the chart and what other planets affected it. Say, Jupiter were in the ascendant in its own sign of Sagittarius. This sign tends to heat things up. Here, it relates to the body so it could incline one towards inflammation and pain. A remedy would be to wear yellow sapphire since it has a cooling effect on Guru. Likewise, if this planet were being cooled too much, a remedy would be to wear yellow topaz which has a warming effect on it.