Birthstone Calculator

A birthstone list is a cultural convention rather than a mineralogical classification: each calendar month is mapped to one or more gem species by an industry standard that has been revised several times. The gems themselves, however, have real and measurable properties. Ruby and sapphire are the same mineral, corundum or aluminium oxide, at 9 on the Mohs hardness scale, differing only in trace chromium versus iron and titanium. Emerald and aquamarine are both beryl at 7.5 to 8. Amethyst and citrine are both quartz at 7, and most commercial citrine is amethyst that has been heated. Diamond, the April stone, sits alone at 10. Those differences drive care requirements. Opal, at 5.5 to 6.5, contains up to about 10 percent water and can craze if dried or shocked; pearl, at 2.5 to 4.5, is calcium carbonate in an organic conchiolin matrix and is attacked by acids and cosmetics, so neither tolerates ultrasonic or steam cleaning that a sapphire shrugs off. Months carrying more than one stone reflect later additions made for supply and affordability rather than antiquity: June has pearl and alexandrite, October opal and tourmaline, November topaz and citrine, and December turquoise, zircon, and tanzanite. The birth flower and zodiac pairings shown alongside come from separate traditions with inconsistent sources, and zodiac dates never align with month boundaries, so nearly every month straddles two signs. For buyers the practical caveat is disclosure, since laboratory-grown corundum and beryl are chemically identical to mined material but far cheaper, and treatments such as fracture filling in emerald are routine.

The birthstone idea is usually traced to the breastplate of Aaron described in Exodus 28, which held twelve engraved stones representing the twelve tribes of Israel. The first-century historian Flavius Josephus linked those twelve stones to the twelve months and the twelve signs of the zodiac, and Saint Jerome, writing around the turn of the fifth century, repeated the association. Collecting all twelve and wearing each in its month was the older custom; wearing only the stone of one's own birth month appears to be a much later practice, often attributed to eighteenth-century Poland and popularized in Britain and the United States during the nineteenth century. The modern list is a trade standard with a documented date. In 1912 the National Association of Jewelers, meeting in Kansas City, adopted a standardized list of monthly stones, an effort strongly influenced by the gemologist George Frederick Kunz of Tiffany and Company, whose book The Curious Lore of Precious Stones appeared in 1913. The Jewelry Industry Council revised the list in 1952, adding alexandrite for June, citrine for November, and tourmaline for October, largely to widen the available price range. Tanzanite, a blue-violet zoisite discovered in northern Tanzania in 1967, was added to December in 2002 by the American Gem Trade Association and Jewelers of America, and the same bodies added spinel as a second August stone in 2016.