In ancient times, people believed that if you knew the location of each planet in the sky at  the exact time that you were born, you would be able to foresee what kind of life you would have.  This is the basis of astrology.  A band of the sky called the zodiac was divided into twelve sections of roughly thirty days each.  Each section had a name - the "Sign of the Zodiac" - and the paths of the planets could be tracked by using these twelve sections as reference points.

When the signs of the zodiac were devised,  long before our modern calendars, the astrological division of the year was based on the equinoxes - the time when the sun crosses the equator and the days and nights are of equal length.  These are approximately 21 March and 23 September.  Also, because the calender year contains 365 days (and 366 in leap years), the signs of the zodiac do not change on the same calendar date each year.  That is why the dates fo zodiac signs are given as approximate, and if you are born on a cusp (the few days at the beginning or end of any sign) your would need to consult astrological tables which calculate the zodiac sign for your date and time of birth.

Each sign of the zodiac has a gemstone associated with it which was supposed to possess special powers and would influence every aspect of a persons life.  This chart shows the stones that are associated with each zodiac sign:

Zodiac Sign Approx Dates Gemstone
Aries  Mar 21 - Apr 19 Diamond
Taurus  Apr 20 - May 20 Emerald / Chrysoprase
Gemini  May 21 - Jun 20 Pearl / Moonstone
Cancer  Jun 21 - Jul 22 Ruby / Carnelian
Leo  Jul 23 - Aug 22 Peridot / Sardonyx
Virgo  Aug 23 - Sep 22 Sapphire / Lapis Lazuli
Libra  Sep 23 - Oct 22 Opal / Tourmaline
Scorpio  Oct 23 - Nov 21 Topaz
Sagittarius  Nov 22 - Dec 21 Turquoise / Zircon
Capricorn  Dec 22 - Jan 19 Garnet
Aquarius  Jan 20 - Feb 18 Amethyst
Pisces  Feb 19 - Mar 20 Aquamarine / Bloodstone