They are gemstone associated with the date of one's birth,
the wearing of which is commonly thought to bring good luck or health.
Supernatural powers have long been attributed by astrologers to certain
gemstones.
The stones now associated with each month have only slight
relationship to the ancient beliefs, for the list is tempered by availability
and cost. Before mineralogy had progressed to the point of chemical analysis,
colour was of greater importance than some of the other physical characteristics,
and little distinction was made between emerald and chrysoprase, for example,
or between ruby and garnet, or between citrine and topaz. When it came to the
ability to heal or bring good luck, the actual stone and the look-alikes were
regarded as equally effective. Even the names used in ancient times do not
necessarily refer to the stones that go by those names in the 20th century; the
sapphire of the Bible is much more likely to have been lapis lazuli than what
is now known as sapphire, and adamas (diamond) was probably white sapphire or
white topaz. (Britannica Encyclopedia)
January: garnet
February: amethyst
March: bloodstone aqumarine
April: diamond
May: emerald
June: pearl
July: ruby
August: sardonyx
September: sapphire
October: opal
November: topaz
December: turquoise