Since Vedic times it has been an accepted belief that gems have curative and prophylactic qualities that influence a man's fate. Hence precious stones served and continue to serve as items of adornment reinforced by a belief in the occult powers. While choosing one’s birthstone it is important to select it after a detailed study of one’s birth-chart, the most important feature being the moon sign or the "Star"/"Nakshatra". One should neither select one’s birthstone according to the month in which one is born nor on the strength of one’s zodiac sign. These are broad classifications and hence may not be very effective. It makes sense to seek the advice of an astrologer who can reveal the best gemstone for an individual.
Generally it is advised to wear a gemstone as per the governance of the ruling planet. This helps to enhance the good qualities indicated by the planet to assure progress, prosperity, physical and mental well being of the person. It is not very advisable to wear a single ring with all the nine stones (nava-ratnam). For example, if one single planet is badly situated in one’s birth chart, its inclusion in the combination could counter the beneficial effects of the other gems. No two stones governed by planets hostile and inimical towards one another should be placed adjacently. Visualize a situation where one who desperately needs a boost in his career wears a wrongly built combination of precious stones, a stone that governs his health prominently. He is sure to be hale and hearty but will continue to struggle throughout his life without achieving any success in his career.
The combination of gems stones is very powerful. It may work out well in the right combination, and if improper the downslide will be sure, subtle and affect family relationships, wisdom, health and happiness. Realization may be too late for redress. People should not fall prey only for prosperity in terms of wealth. If you wish to wear a gemstone to enhance your fortune, do not select the stone according to the jeweller's list. Even though the gemstones for each month are universally displayed at various jewellery stores, these do not have astrological sanctions. According to Vedic astrology there are nine gems governing the nine planets.
Mars - Coral; Mercury - Emerald; Jupiter - Yellow Sapphire; Venus - Diamond; Saturn - Blue Sapphire; Sun - Ruby; Moon - Pearl; Rahu, - Gomeda (Hessonite) and Ketu - Cat's Eye.

Gems like Amethyst, Aquamarine, Bloodstone, Moonstone, Garnet, Peridot, etc., do not have any protective value according to Vedic astrology. Choose the gemstone as per your birth star and wear on the day auspicious to your ruling planet.