A birthstone is a gemstone or other semi-precious stone which is associated with a month of the Gregorian Calendar. There is no particular connection between the stones and the months, but they are often used as a way to "personalize" jewelry, taking the stone for the month in which a person was born.
Here is a chart of the months with corresponding stones: www.birthday-horoscopes.net
Here is a chart of the months with corresponding stones: www.birthday-horoscopes.net