Birthstones are a fascinating aspect of the gem and jewelry world, and are derived from early beliefs regarding the one’s time of birth and its relationship to the planets. Wearing a certain stone as protection against illness and misfortune, or another gem for good luck, eventually developed into the birthstone systems of today.

Modern Birthstones
From these early systems, comes the modern birthstone system.

The Western birthstone system originates from the writings of Josephus (1st century AD) and St. Jerome (5th century AD). Both trace the custom to the gems of the Hebrew High Priest’s Breastplate. Over time, the birthstone custom died, only to be revived again in 18th-century Poland.

Zodiac Gems
Long before the modern 12-month calendar was invented, astrologers assigned certain gemstones to the 12 signs of the zodiac based on the symbolism and metaphysical powers each stone was believed to possess.

Sign	Sign	Zodiac Stone		Sign	Sign	Zodiac Stone
	Capricorn Dec. 22–Jan. 20	Ruby			Cancer June 22–July 22	Emerald
	Aquarius Jan. 21–Feb. 21	Garnet			Leo July 23–August 22	Onyx
	Pisces Feb. 22–March 21	Amethyst			Virgo August 23–Sept. 22	Carnelian
	Aries March 22–April 20	Bloodstone			Libra Sept. 23–Oct. 23	Peridot
	Taurus April 21–May 21	Sapphire			Scorpio Oct. 24–Nov. 21	Beryl
	Gemini May 22–June 21	Agate			Sagittarius Nov. 22–Dec. 21	Topaz

Indian Planetary Gemology
In ancient India, beliefs regarding the planets and their relationships with stones also is found. This has developed into what some today refer to as “planetary gemology.” In such systems, there are nine planets and nine gems, as shown below. Sometimes jewelry is made using one each of these gems. These are termed navaratna jewels (nava = nine; ratna = jewel).

Sign	Planet	Hindu Planetary Stone		Sign	Planet	Hindu Planetary Stone
	Sun	Ruby			Venus	Diamond
	Moon	Pearl			Saturn	Blue Sapphire
	Mars	Coral			Rahu (moon’s north node)	Hessonite Garnet
	Mercury	Emerald			Ketu (moon’s south node)	Cat’s Eye
	Jupiter	Yellow Sapphire

A fine modern example of a navaratna jewel, based on Indian planetary gemology. The gems are clockwise from the top, diamond, pearl, coral, hessonite garnet, sapphire, cat’s eye, yellow sapphire, emerald and ruby (center). Jewel and photo courtesy of Richard Brown, Astral Gemstones and Talismans, Bangkok, Thailand.

Birthstones for days of the week
Another method of choosing a birthstone is according to the day of the week upon which one is born. In his book The Curious Lore of Precious Stones, George Kunz notes that specific gemstones are also associated with the day of the week you were born. These are listed below, along with alternatives of comparable hue for the more unusual gemstones.
The week is a division of time equal to seven days. We do not know exactly where it comes from, but the ancient Hebrews were among the first to use it. The book of Genesis in the Bible says that the world was created in six days and the seventh day, or Sabbath, was a day of rest and worship.
Ancient Egyptians named the days after planets, which they incorrectly believed included the sun and the moon. The seventh day was considered merely a day of rest and play. In ancient Rome, the days of the week were named after the sun, the moon, and the five planets then known. Each day was considered sacred to the Roman god associated with that planet. The days were known as Sun’s-day, Moon’s-day, Mars’-day, and so on. This system was used about the beginning of the Christian Era. The English names for the days Tuesday, Wednesday, Thursday, and Friday were derived from the names of Norse gods.
Most Latin-based languages connect each day of the week with one of the seven “planets” of the ancient times: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. French, for example, uses the following:

Gems for all seasons
One can also select gems by the season, as follows:

Of course, many countries do not have four seasons (Thailand, etc.), but we won’t get into that.

Choosing a birthstone
If nothing else, the above should make clear that there are a bewildering variety of birthstone choices, depending on nationality, culture, religion, etc. Which one is right for you? We suggest that, if none of the above birthstones strikes your fancy, you simply choose a personal gemstone that symbolizes a special time for you. Choose a personal gemstone based on your zodiac sign, the day of the week on which you were born or the color designated for your birth month.

Notes on Calendars
Before the invention of the clock, people watched the sun, moon and stars to tell time. The daily rising of the sun provided a short unit of time, the solar day. The cycle of seasons roughly indicated a longer unit of time, the solar year. But early people did not know that the earth’s revolution around the sun caused the different seasons. The changing position and shape of the moon was easier for them to observe. As a result, early calendars used the interval between the successive full moons, called the lunar month, as an intermediate unit of time.

Did you know? Did you know that ruby was so esteemed by the Burmese that many goldsmiths in that country would refuse to mount it as a side stone? This star ruby recently sold by Pala International is a particularly fine example of just why Burmese rubies are in a class by themselves.

Today we know the lunar month lasts about 29 1/2 days. Twelve such months amount to about 354 days. This interval is almost 11 days shorter than the true solar year, which has 365 days, 5 hours, 48 minutes, and 46 seconds. But a year of 13 lunar months would equal about 383 1/2 days and would be more than 18 days longer than the solar year. The solar year, therefore, does not equal any whole number of lunar months.
Note that these numbers are averages. The actual length of a particular year may vary by several minutes due to the influence of the gravitational force from other planets. Similarly, the time between two new moons may vary by several hours due to a number of factors, including changes in the gravitational force from the sun, and the moon’s orbital inclination.
The discrepancy between whole lunar months and days in a solar year explains the confusion over calendar keeping during thousands of years. A calendar based on 12 lunar months becomes out of step with the seasons. Some people who used lunar calendars kept them roughly in step with the seasons by making some years 12 months long and other years 13 months long.
Early calendars usually represented some sort of compromise between the lunar and solar years. Some years lasted 12 months, and others lasted 13 months.
The Christian calendar (Gregorian calendar) is based on the motion of the earth around the sun, while the months have no connection with the motion of the moon.
On the other hand, the Islamic calendar is based on the motion of the moon, while the year has no connection with the motion of the earth around the sun.
Finally, the Jewish calendar combines both, in that its years are linked to the motion of the earth around the sun, and its months are linked to the motion of the moon.

Further reading

• Kunz, G.F. (1913) The Curious Lore of Precious Stones. J.D. Lippincott, 406 pp.
• Click on these links more information on the origin of calendars and days of the week