October is one of the few birth months assigned two official gemstones: opal and tourmaline. The pairing dates back to the early 20th century, when the National Association of Jewelers (now Jewelers of America) standardized the modern birthstone list in 1912 and included both stones for October.
Opal is known for its play of color, a shifting rainbow effect unlike any other gem. Tourmaline is known for the widest color range of any gemstone family, including bicolor and tricolor crystals. This guide covers the science, history, and buying considerations for both.
Opal: The Science Behind Play of Color
Opal is not a crystalline mineral in the traditional sense. It is a hydrated form of silica, meaning it contains water trapped within its structure, typically 6 to 10 percent by weight. Unlike diamond or sapphire, opal has no fixed crystal lattice, which is why gemologists classify it as a mineraloid rather than a true mineral.
Opal's signature effect, called play of color, comes from microscopic silica spheres arranged in a grid pattern within the stone. When these spheres are uniform in size and evenly spaced, light diffracts as it passes through, splitting into flashes of color across the surface. The size of the spheres determines which colors appear, smaller spheres produce blue and violet flashes, while larger spheres produce red and orange.
On the Mohs hardness scale, opal ranks between 5.5 and 6.5, making it noticeably softer than sapphire or diamond. This affects how it should be set and worn.
Common Opal Varieties
- White opal: Light background with visible play of color
- Black opal: Dark background that makes color flashes appear more vivid, considered the rarest and most valuable
- Fire opal: Orange to red background, sometimes without play of color
- Boulder opal: Formed with ironstone still attached to the back
At Fascinating Diamonds, opal pieces are lab-grown, replicating the same silica sphere structure responsible for natural play of color while offering more consistent clarity. Browse our gemstone jewelry collection to explore opal alongside other birthstone options.
Tourmaline: The Science Behind Its Color Range
Tourmaline is a crystalline borosilicate mineral, and it holds the record for the widest color range of any gemstone species. This is due to variations in trace elements such as iron, manganese, chromium, and titanium during crystal formation. Because tourmaline crystals often grow with changing mineral conditions over time, a single crystal can display multiple colors, a variety known as bicolor or watermelon tourmaline when pink and green appear together.
On the Mohs scale, tourmaline ranks 7 to 7.5, making it more durable than opal and suitable for rings intended for daily wear.
Common Tourmaline Varieties
| Variety | Color | Cause |
|---|---|---|
| Rubellite | Pink to red | Manganese |
| Indicolite | Blue | Iron and titanium |
| Chrome tourmaline | Green | Chromium and vanadium |
| Paraiba | Neon blue-green | Copper |
| Watermelon | Pink core, green rim | Changing mineral conditions during growth |
Paraiba tourmaline, first discovered in Brazil in the late 1980s, remains one of the most valuable colored gemstones due to its saturated neon hue and limited supply.
History and Symbolism
Opal has been associated with hope, purity, and truth since antiquity. Ancient Romans considered it a symbol of hope and good fortune, and it was often carried as a protective talisman. Its shifting colors have historically been linked to inspiration and creativity.
Tourmaline entered Western gem trade later than opal, gaining prominence in the 18th and 19th centuries after Dutch traders brought crystals from Sri Lanka to Europe. It became associated with balance, calm, and emotional grounding, partly due to a historical (and now scientifically explained) property: tourmaline is pyroelectric and piezoelectric, meaning it generates an electric charge when heated or compressed. This unusual property added to its mystique in earlier centuries.
Choosing Between Opal and Tourmaline
Since October offers a choice between two visually distinct gems, the decision often comes down to durability needs and desired aesthetic.
Opal is a better fit when:
- The piece will be worn occasionally rather than daily, such as earrings or a pendant
- The wearer wants the unique play of color effect that no other gem replicates
- A lighter, more ethereal look is preferred over a single solid hue
Tourmaline is a better fit when:
- The piece is intended for daily wear, including rings
- A specific, consistent color is preferred over a shifting effect
- The wearer wants a wider range of saturated hues to choose from, from deep pink to neon green
How to Choose a Piece of Either Gemstone
For Opal
- Play of color intensity: Stronger, more vivid flashes across a wider range of the visible spectrum increase value.
- Background color: Black and dark opal backgrounds make play of color more visually striking than white or light backgrounds.
- Pattern: Broad, evenly distributed color patches are generally valued over small, scattered flecks.
- Setting: Bezel settings are recommended over prong settings, since opal's lower hardness makes it more prone to chipping on exposed edges.
For Tourmaline
- Color saturation: Medium-to-deep, evenly distributed color without visible zoning is preferred.
- Clarity: Tourmaline typically forms with fewer inclusions than opal, so eye-clean stones are the standard expectation.
- Cut: Since tourmaline can show different color intensity depending on viewing angle (a property called pleochroism), cut orientation affects the final face-up color.
- Carat weight: Larger tourmaline crystals are more available than large opals, making bigger carat weights more accessible at lower price points.
Caring for Opal and Tourmaline Jewelry
Opal:
- Avoid ultrasonic cleaners and steam cleaning, both of which can cause cracking due to opal's water content.
- Clean only with a soft cloth and lukewarm water.
- Avoid prolonged exposure to dry heat or direct sunlight, which can cause dehydration cracking over time.
- Store away from other jewelry to prevent surface scratches.
Tourmaline:
Real Customer Reviews
Frequently Asked Questions
Why does October have two birthstones?
The modern birthstone list was standardized in 1912 by the National Association of Jewelers. October was assigned both opal and tourmaline to offer a wider range of options, since the two gems differ significantly in appearance and durability.
Is opal a durable gemstone for daily wear?
Opal ranks 5.5 to 6.5 on the Mohs hardness scale, softer than most gemstones used in daily wear jewelry. It is better suited to occasional-wear pieces like earrings or pendants rather than rings worn every day.
What causes tourmaline's wide color range?
Trace elements such as iron, manganese, chromium, and copper enter the crystal structure during formation, and shifting mineral conditions can even produce multiple colors within a single crystal.
What is watermelon tourmaline?
Watermelon tourmaline is a bicolor variety with a pink core and green outer rim, resembling the cross-section of the fruit. It forms when mineral conditions change during the crystal's growth.
Is lab-grown opal the same as natural opal?
Lab-grown opal replicates the same silica sphere structure responsible for natural play of color, offering comparable optical effects with more consistent clarity.