Opal is a gemstone of extraordinary visual phenomenon and deep historical roots. As the traditional birthstone for October, it is celebrated for its unique "play-of-color," a spectral display that can flash every color of the spectrum with an intensity that, according to some sources, surpasses the fire of a diamond. Its composition, a hydrated variety of amorphous silica, places it in a distinct category known as a mineraloid rather than a true mineral. This fundamental structure is responsible for both its breathtaking beauty and its notable fragility, requiring careful consideration in jewelry design and wear. Beyond its physical properties, opal is woven into a rich tapestry of folklore, viewed across cultures as a symbol of hope, purity, and truth, and attributed with a range of spiritual and healing properties. This article delves into the science, history, and cultural significance of opal, exploring its geological origins, gemological characteristics, and its enduring role in human adornment and belief.
History and Lore
The history of opal is as ancient and colorful as the stone itself. The name "opal" is believed to derive from the Latin Opalus, meaning "precious stone," and the Greek Opallios, meaning "to see a change in colour," both of which are traced back to the older Sanskrit word upala, which translates to "stone" or "jewel." The term upala dates to approximately 250 BC, a period when opal was considered more valuable than any other gemstone in the world. Archaeological evidence points to the early use of opal, with finds in North America dating back over 10,000 years and in Kenya around 6,000 years ago. The Kenyan discovery is thought to have originated from Ethiopian deposits, as Ethiopia borders Kenya to the north.
During Roman times, the major sources of opal were believed to be forgotten, and the only noted source was in Cervenica, Slovakia. For centuries, opal was revered as the most precious of gems because it was thought to harbor the colors of all other revered gemstones within its single stone. This perception of opal as a composite of all gem colors contributed to its mystique and high value. The stone is also linked to supernatural origins in various legends, with some cultures viewing it as a symbol of hope, purity, and truth, and even believing it had fallen from the heavens.
Opal's journey to becoming a modern birthstone is rooted in tradition. It is the designated birthstone for the month of October and is also associated with the zodiac sign of Libra, covering birthdays from September 23rd to October 22nd. Its status as a birthstone is well-established, though its reputation has fluctuated. Some scholars suggest that many ancient references to opal may have actually described other iridescent gems, such as iris agate, which could have contributed to the stone's legendary status. Despite these historical complexities, opal's captivating appearance has ensured its place as a treasured gem throughout human history.
Geological Formation and Sources
Opal is a sedimentary stone formed under specific geological conditions. Its formation begins with water percolating through the earth, where it becomes rich in dissolved silicates. When this silicate-rich water enters a cavity, it deposits the silicates as microscopic spheres. The arrangement and uniformity of these spheres determine the type of opal formed. If the spheres are uniform in size and shape and neatly stacked, they diffract light, creating the precious opal known for its play-of-color. If the spheres are random in size, shape, and arrangement, the result is common opal, which lacks the spectral fire of its precious counterpart.
Opal deposits are found in several regions around the globe. Historically, the only noted source in Roman times was in Cervenica, Slovakia. However, significant discoveries in Australia in the late 19th century transformed the market. Australia is now the primary source of precious opal, with its deposits being exceptionally rare and almost exclusively Australian. The Australian deposits, particularly those in regions like Lightning Ridge and Coober Pedy, are famed for producing some of the world's finest black opals and crystal opals. Beyond Australia, other notable sources include Ethiopia, which has gained prominence in recent years for its hydrophane opals, and the United States, where opal has been found in states like Nevada and Idaho. The historical discovery in Kenya, thought to be of Ethiopian origin, underscores the long-standing presence of opal in East Africa.
The geological conditions required for precious opal formation are quite specific, involving the right balance of silica-rich water and stable conditions to allow for the slow deposition of spheres. This precise formation process contributes to the rarity and value of high-quality precious opal, commanding prices per carat that can rival expensive diamonds, rubies, sapphires, and emeralds.
Gemological Properties
Opal is unique among gemstones due to its amorphous structure. Mineralogically, it is a hydrous silicon dioxide with a chemical composition of SiO2·nH2O. The "n" in the formula indicates a variable amount of water, typically ranging from 3% to 21%, with gem-grade opals usually containing 6% to 10% water. Because it lacks a crystalline structure and has a variable chemical composition, opal is classified as a "mineraloid" rather than a true mineral. This amorphous nature is fundamental to its characteristics and distinguishes it from crystalline gems like quartz.
The visual appeal of opal is defined by its color and optical phenomena. Common opal exhibits a bodycolor that can be white, yellow, gray, black, or brown, though it can occur in virtually any color, including red, orange, pink, green, and blue. Precious opal is distinguished by its "play-of-color," a spectral display of red, orange, yellow, green, blue, and violet that flashes across the stone as it is moved. This play-of-color is not an iridescence but a result of diffraction, caused by the interference and scattering of light by the uniform microscopic silica spheres within the stone. The luster of opal varies; rough opal can have a dull, pearly, waxy, or vitreous luster, but most opal polishes to a vitreous (glass-like) luster. In terms of diaphaneity, opal can be translucent, opaque, semi-transparent, or, more rarely, transparent. It has no cleavage and breaks with a conchoidal fracture.
A critical gemological property of opal is its hardness. Opal has a hardness of approximately 5.5 to 6.0 on the Mohs hardness scale. This relatively low hardness makes it softer and more easily chipped than many other popular gemstones, such as quartz (hardness 7) or diamond (hardness 10). The streak of opal is white. These properties are summarized in the table below.
Table 1: Key Gemological Properties of Opal
| Property | Description |
|---|---|
| Chemical Classification | A hydrated variety of amorphous silica (SiO2·nH2O) |
| Color | Bodycolor: white, yellow, gray, black, brown, or any color. Precious opal displays play-of-color (spectral colors). |
| Streak | White |
| Luster | Dull, pearly, waxy, or vitreous (polishes to vitreous) |
| Diaphaneity | Translucent, opaque, semi-transparent, rarely transparent |
| Cleavage | None; breaks with conchoidal fracture |
| Hardness (Mohs) | 5.5 – 6.0 |
Symbolism and Metaphysical Beliefs
Opal has been attributed with a wide array of symbolic meanings and metaphysical properties throughout history, contributing to its enduring allure. Culturally, it is widely viewed as a symbol of hope, purity, and truth. Its otherworldly appearance has often been linked to supernatural origins, with some legends stating it fell from the heavens. This association with the cosmos and the divine has made it a stone of great spiritual significance.
In holistic and metaphysical traditions, opal is believed to possess powerful healing and emotional properties. Holistic healers use opal to intensify emotions and cosmic consciousness. It is frequently said that wearing opal enhances creativity and intelligence in the wearer and heightens a connection to oneself. Some beliefs extend further, suggesting that opal can strengthen recall and memory. In terms of physical healing, opal is thought to treat infections and fevers while simultaneously purifying the liver and kidneys. It is also considered an excellent stone for women, with claims that it can alleviate menstrual cramps and ease the pain of childbirth. These metaphysical and healing properties are based on holistic health theories that have persisted over time, with many people turning to opal for its perceived benefits to physical, mental, and emotional well-being.
It is important to note that these beliefs are part of a long-standing tradition of crystal healing and cultural lore. While these properties are widely cited in gemstone literature and by holistic practitioners, they are not established by empirical scientific evidence. The value of these beliefs lies in their cultural and personal significance to those who ascribe to them.
Care and Cleaning
Given its unique physical properties, opal requires specific care to maintain its beauty and integrity. Its relatively low hardness (5.5 to 6.0 on the Mohs scale) and lack of cleavage make it susceptible to scratching, chipping, and abrasion. Furthermore, opal is sensitive to moisture and sudden changes in temperature. This fragility dictates how and where it should be worn in jewelry.
Opal is best suited for jewelry items that do not receive significant impact or abrasion during wear, such as earrings, pendants, and brooches. These settings protect the stone from daily knocks. While opal rings are beautiful and popular, they require extra caution. For ring use, the best designs employ a bezel setting that fully protects the stone, rather than a prong setting that exposes it. If an opal ring is worn, it is advisable to remove it during activities where impact or abrasion might occur, such as gardening, sports, or manual work. Some sources also recommend reserving opal rings for occasional wear rather than daily use.
Cleaning opal must be done gently. Due to its water content and potential porosity (especially in hydrophane varieties from Ethiopia), opal should not be soaked in water or exposed to harsh chemicals, ultrasonic cleaners, or steam. The safest method is to wipe the stone with a soft, damp cloth and dry it immediately with a soft, dry cloth. Avoid sudden temperature changes, as thermal shock can cause cracking. For opals in protective settings like bezels, regular gentle cleaning is sufficient to maintain their luster and appearance.
Conclusion
Opal stands as a gemstone of unparalleled beauty and complexity. As the traditional birthstone for October, it carries the weight of ancient history, from its Sanskrit origins as upala to its revered status in Roman times. Its formation as a sedimentary mineraloid, composed of microscopic silica spheres, results in the mesmerizing play-of-color that defines precious opal and distinguishes it from all other gems. While its hardness of 5.5 to 6.0 necessitates careful handling and protective jewelry settings, its visual impact makes it a favored choice for earrings, pendants, and brooches. Beyond its physical attributes, opal is deeply embedded in human culture as a symbol of hope and truth, and it is attributed with a rich array of spiritual and healing properties in holistic traditions. From the ancient deposits of North America and Kenya to the renowned fields of Australia, opal continues to captivate gemstone enthusiasts, jewelry buyers, and students of gemology with its unique combination of scientific wonder and cultural legacy.