Modern Birthstones

The most commonly used birthstone system (based on the birth month) has nothing to do with astrology and signs of the Zodiac.

Since the signs of the Zodiac change anywhere from the 19th to the 22nd of each month, a list of Zodiac birthstones cannot be based on the months of the calendar year.
 

According to the Tropical system of astrology prevalent in the West, zodiac or birth signs are determined by the position of the Sun in one's horoscope. In the Sidereal system this is determined by the position of the Moon. But in both systems, the zodiac sign is important, not the month of one's birth. Each sign of the zodiac (rasi) is ruled by one of the planets and each planet in turn rules over certain gemstones according to their color.