The relationships between magical gemstones, minerals, crystals, and semi-precious stones and the months of the year actually has its roots in Astrology, as it was the custom from classical times onward to associate both metals and gems with the seven sacred planets, and thus with the zodiacal signs they rule.
In modern times, zodiacal or seasonal religious calendars have been replaced by secular calendars, and by the 19th century CE, the minerals were discarded and the gemstones were assigned to various months. Some of these designations allied a month with a partial zodiac sign, and thus with its gemstone, but some of the stated stones were placed in their secular months by arbitrary taste.
The history of how secular months came to be represented by gemstones is told in the page on The Tiffany Birthstone Poem.
Because many of the vintage postcards at this site that correlate months of the year with gemstones also contain corresponding flowers and zodiac signs, you may also find them double- or triple-listed in the categories of Floriography and Astrology.
Read all about lucky stones and their use in fortune telling in "Crystal Magic: Divination, Healing, and Spellcraft with Gems and Minerals" by Reverend Jon Saint Germain:
curator, historian, and docent
Your Wate and Fate
Special thanks to my dear husband and creative partner nagasiva yronwode for illustrations, scans, and clean-ups.