Opal is the October birthstone – and its play-of-color is so mesmerizing that it borders on magical. From coveted black opal, to serene water opal, to blazing fire opal, there’s one for every taste. We help you find your favorite.
In this post, we cover:
October Birthstone: Gemological Properties of Opal
There are two broad classes of opal: precious and common. When the October birthstone is precious opal, it displays play-of-color (the breaking up of waves of light into colors of the rainbow). Common opal does not. Opal is made of billions of uniform submicroscopic silica spheres, stacked in an orderly pattern. The size of the spheres and the way they are packed determines which colors will be produced (if any) and their intensity.
How opal forms is still something of a mystery. No single scientific model has been able to account for the many different opal deposits around the world and different opal types. One model, the weathering model, is the most well known and easily understood. In this model, opal forms in desert areas that have strong seasonal rainfall and rocks rich in silica (a compound of silicon and oxygen), such as volcanic ash and some sedimentary rocks. During winter, heavy rains raise the water table. As rainwater trickles down through the rocks, it carries the silica with it. The silica-rich solution fills fractures and cavities in rocks below the water table. In summer, the desert sun dries the landscape and the water table retreats downward. As the water evaporates, it leaves spheres of silica in place. This process is repeated year after year – for millions of years – until the layers of silica spheres form opal.
Gemologists love opal because its flashing play-of-color is caused by diffraction of light by the silica spheres, which are stacked like tiny Ping-Pong balls in a box. They also love the endless variety of patterns and colors in opal: No two opals are exactly alike.
Pattern is the arrangement of an opal’s play-of-color. The industry categorizes opal’s play-of-color patterns into three types:
- Pinfire – Very small patches or “dots” of color
- Flash – Large areas of color
- Harlequin – Large, distinct, usually rectangular patches of color with edges touching each other. The harlequin pattern is extremely rare and considered by many to be the most valuable.
GIA classifies the many varieties of opal into seven categories:
- Black opal: Background color typically ranges from translucent to opaque black to dark gray, but can also be blue, green or brown that appears black in reflected light; can show exceptional play-of-color.
- White opal: Background color ranges from translucent white to medium gray; shows play-of-color.
- Crystal opal: Background color ranges from transparent to semitransparent; shows good play-of-color.
- Water opal: Background color ranges from transparent to translucent and can be colorless, white, purple, brownish, blue or blue-green; shows some or no play-of-color.
- Fire opal: Background color ranges from transparent to translucent and can be yellow, orange, reddish orange or red; may or may not show play-of-color.
- Boulder Opal: Includes part of the host rock, or matrix, in the finished gem; shows play-of-color.
- Assembled opal: Layers of gem opal, or of opal and other materials, cemented together to improve durability and appearance. Two types of assembled opal are doublets (a thin layer of opal cemented to a backing of dyed black chalcedony, glass, common opal or matrix) and triplets (a thin layer of opal cemented between a domed top of colorless quartz or clear glass and a backing of dyed black chalcedony, common opal or glass).
October Birthstone: Opal History & Lore
The October birthstone’s dramatic play-of-color has inspired writers to compare it to fireworks, galaxies and volcanoes. Bedouins also had a colorful theory: They believed opal held lightning and fell from the sky during thunderstorms. Ancient Greeks thought opals bestowed the gift of prophesy and protection from disease. Europeans long maintained opal to be a symbol of purity, hope and truth. Hundreds of years ago, opal was believed to embody the virtues and powers of all colored stones.
Yet opal has also been dogged by negative associations. These superstitions may have started with gem cutters, who were frustrated by the tendency of opal to fracture during cutting and setting. Since lapidaries were typically held responsible for any damage to a stone, they may have spread the word that opal was unlucky. A misinterpretation of Sir Walter Scott’s novel Anne of Geierstein also contributed to the widespread belief that the gem was cursed, as the opal Lady Hermione wore played a role in breaking the spell that dissolved her enchantment.
We give the Roman scholar Pliny (23–79 AD) the last word on opal’s allure: “There is in them a softer fire than in the carbuncle [at the time, any of several red gemstones], there is the brilliant purple of the amethyst; there is the sea-green of the emerald – all shining together in incredible union. Some by their refulgent splendor rival the colors of the painters, others the flame of burning sulfur or of fire quickened by oil.”
The Sun God Opal in the gem collection at Chicago’s Field Museum is famous for its uniqueness and provenance. A human face has been carved into the 35 ct cabochon, which is surrounded by gold flames. Once owned by the Hope family, of Hope Diamond fame, the Sun God Opal was reputedly discovered in Mexico in the 16th century.
October Birthstone: Where Opal Comes From
Opal is found in many places. The fields of Australia are the most productive in the world for the October birthstone. Ethiopia, Mexico and Brazil are also important sources. Additional deposits have been found in Central Europe, Honduras, Indonesia, Madagascar, Peru, Turkey and the United States.
Lightning Ridge, a small town in New South Wales, Australia, is famed for producing prized black opal. A dry and rocky place softened only by small trees and scrub brush, Lightning Ridge gets little rain and bakes in the scorching summer temperatures. The climate is so unforgiving that miners often live in underground housing to find respite from the punishing heat.
Australia is also a source of other types of the October birthstone. White opal is found in the White Cliffs area of New South Wales, as well as in Mintabie, Andamooka and Coober Pedy in South Australia. Boulder opal, which is found in only one location in the world, is mined in Queensland.
In Ethiopia, the October birthstone is found near the village of Wegel Tena, in Wollo Province. Travel 340 miles north of the capital Addis Ababa and up 8,000 feet, where miners pry opal from shafts dug into the side of a plateau. Gems unearthed here range in body color from white, yellow, orange and brownish red to “chocolate” brown. Some of the opals show play-of-color. Another mine, in Ethiopia’s Shewa Province, yields the coveted dark and black opal, as well as orange, white and crystal opal. Its treasures are buried in steep cliffs that tower over the landscape.
Querétero, a state in Mexico, is known for producing fire opal in yellow, orange and reddish orange to red, some with good play-of-color. The mines are a tourist destination, and getting to them requires taking a dirt road through dense forests of pine and oak, past scrubby plateaus of cacti and shrubs, and up winding mountain roads. The city of Querétaro is worthy of the October birthstone: Its stately colonial architecture and lively plazas have made it a UNESCO world heritage site.
White opal is found in Piauí State, in northeastern Brazil. The town of Pedro Segundo, known for its annual jazz and blues festival, is near the mines. Small trees and brush color the landscape. The destination is remote and hard to reach.
October Birthstone: Qualities to Look for in Opal
Purchasing an opal is similar to buying a work of art: It is a matter of taste. Determining an opal’s value is also highly subjective, and trade members often disagree about which characteristics raise or lower an opal’s value. Still, there are some guidelines you can use to make a smart purchase.
Color: The combination of background color and play-of-color creates opal’s color.
- Background color is the basic hue of the opal. Some background colors tend to be more prized than others. All other quality factors being equal, many buyers favor black. This is partly because play-of-color tends to stand out attractively against a dark background.
- Play-of-color is an opal’s unique display of flashing rainbow colors. Opal lovers prize bright play-of-color over faint play-of-color. The most valuable opals display play-of-color from all angles, across the entire stone, in the full range of hues from red through blue. Play-of-color hues should contrast with the background color.
Clarity: With an opal, clarity is its degree of diaphaneity and absence, or lack, of inclusions or blemishes. An opal’s diaphaneity can range from transparent to opaque. Experts prize different levels of diaphaneity for different opal types. Crazing (see below) and pits are examples of problem inclusions and blemishes for opal.
Cut: The cutter considers an opal’s color, clarity and pattern when planning the finished gem. In general, connoisseurs prefer large, closely arranged patches of color over tiny, scattered dots.
As with many top-quality colored stones, exceptional opals may not be cut to standard sizes and shapes. Domed surfaces, referred to as cabochons, display the best play-of-color.
Carat Weight: Opals come in a wide range of sizes. However they are commonly cut into oval cabochons in calibrated sizes such as 6 x 4, 7 x 5, 8 x 6 and 10 x 8 millimeters that can be easily set in jewelry.
October Birthstone: Opal Treatments, Care & Cleaning
Opal can be treated by impregnation with oil, wax or plastic, and by surface modifications called sugar treatment and smoke treatment, in order to enhance its play-of-color or background color.
Untreated opal is generally stable, but heat from intense light can cause fracture lines called crazing. High heat or sudden temperature changes can also cause opal to fracture. Hydrofluoric acid and caustic alkaline solutions will damage the October birthstone. Do not store opal in a dry place, as lack of moisture may damage the stone, too. A soft, padded, cloth bag is a good choice; a safety deposit box is less than ideal (it is a dry environment).
The safest way to clean the October birthstone is with warm, soapy water. Other cleaning methods might damage the opal or filler material. Note that prolonged exposure to water may weaken the adhesive in opal doublets and triplets.
Opal ranges from 5 to 6.5 on the Mohs scale of hardness. To prevent jewelry set with gems that are harder from scratching opal, store it by itself. Diamonds, rubies, sapphires and emeralds are just a few of the gems that can scratch the October birthstone.
Infatuated with the October Birthstone? Our Opal Buying Guide can help you pick an enchanting one.