Birthstone for October: Opal.

Multi-Colored · Hope, creativity, innocence

Overview & Meaning

October illuminates autumn with opal, a gemstone celebrated for its mesmerizing 'play-of-color'—a dancing rainbow of shifting hues that seems to capture the beauty of the cosmos. Formed from microscopic silica spheres arranged in regular three-dimensional grids, precious opal diffracts light into flashes of fiery red, electric green, vivid violet, and turquoise blue. Ancient cultures revered opal as a stone of pure wonder, containing the virtues of every colored gemstone in a single matrix.

Sharing October is tourmaline, an exceptionally diverse mineral species officially added to the birthstone list in 1952 by the Jewelry Industry Council. Renowned for displaying an unmatched spectrum of colors—from the two-toned 'watermelon' tourmaline (green rind with pink core) to electric Paraíba blues—tourmaline represents artistic expression, emotional empathy, and boundless imagination. Together, October’s birthstones encourage bold self-expression, creative inspiration, and joyful optimism.

Mineralogical & Geological Profile

Opal

Chemical Formula

SiO2 · nH2O (Hydrated amorphous silica, 3–21% water)

Mineral Species / Class

Mineraloid (Opal) / Complex Cyclosilicate (Tourmaline)

Mohs Hardness

5.5 – 6.5 (Opal) / 7.0 – 7.5 (Tourmaline)

Crystal System

Amorphous (Opal) / Trigonal (Tourmaline)

Geological Formation & Origin

Precious opal forms in arid sedimentary basins (most famously the Great Artesian Basin in Australia) when seasonal rains wash silica-rich groundwater down into sandstone crevices. Over millions of years, the water evaporates, leaving behind tightly packed silica spheres. When these spheres measure between 150 and 300 nanometers and align in periodic arrays, they act as natural diffraction gratings, splitting white light into vibrant spectral colors.

History, Lore & Symbolism

Ancient Romans prized opal above all gems; Pliny the Elder praised it for uniting the deep fire of the carbuncle, the purple of amethyst, and the sea-green of emerald. However, a 19th-century superstition arose following Sir Walter Scott's 1829 novel Anne of Geierstein, in which a heroine dies shortly after holy water dulls her enchanted opal talisman—a fictional plot that severely harmed European opal sales for decades. That fear was shattered with the 1915 discovery of vast precious opal fields in Coober Pedy, Australia by 14-year-old William Hutchison, cementing Australia as the opal capital of the world. Tourmaline was formally added in 1952 as an October alternative.

Durability, Care & Cleaning

Opal is a delicate, porous mineraloid with a Mohs hardness of 5.5 to 6.5 and a water content typically between 3% and 10%. Keep opals away from ultrasonic machines, boiling water, steam cleaners, and harsh chemicals, which can cause cracking (crazing). Clean gently with a soft cloth dampened with mild soapy water. Tourmaline is significantly tougher at 7.0 to 7.5 Mohs, handling daily wear with greater resilience.

Frequently Asked Questions

Q1 Why does opal flash with different colors?

Opal’s 'play-of-color' is caused by diffraction: microscopic spheres of silica arranged in an orderly lattice bend and split white light into the spectral colors of the rainbow, with sphere size determining the colors visible.

Q2 Is it true that opals bring bad luck?

No, that is an 1800s literary myth stemming from Sir Walter Scott's 1829 gothic novel Anne of Geierstein. Historically, Romans, Greeks, and medieval Europeans considered opal the luckiest of all gemstones because it held the colors of every gem.