Birthstones are gemstones that are associated with each month in the Gregorian Calendar. They may be precious stones - those rare and expensive gems such as diamonds, emeralds, rubies, pearls, and sapphires - or semi-precious, which includes practically all the others such as ameythst, opal and bloodstone.
In each case they are considered lucky for their particular months and their healing powers are allegedly heightened during their months. Thus, if you are born in January, then your birthstone is the garnet, and you will get the most benefit from this stone by wearing it in January.
However, as each birthstone has heightened powers during its month, some people will buy a set of all twelve and rotate them for each month.