October’s birthstone is opal.
Opal is the October birthstone. Some people think it’s unlucky for anyone born in another month to wear an opal, which is a superstition that comes from a novel written in the 1800s (Anne of Geierstein by Sir Walter Scott), and not from any ancient belief or experience. In fact, throughout most of history, opal has been regarded as the luckiest and most magical of all gems because it can show all colors. At one time, opal was thought to have the power to preserve the life and color of blond hair.
- MINERAL:Hydrated Silica
- COLOR:All colors
- REFRACTIVE INDEX:37-1.47
- SPECIFIC GRAVITY:15 (+0.08, -0.90)
- MOHS HARDNESS:5 to 6.5
When opal formed, silica gel filled crevices in rock. As water evaporates, the silica is deposited in the form of tiny spheres. Opal contains up to 20% water trapped in its silica structure.
Opal’s arrays of silica spheres, which are like pin-pong balls stacked in a box, form a fantastic variety of patterns and flashing colors by diffraction of light by the silica spheres. No two opals are exactly alike. Grids of silica spheres 0.2 microns in size create red play-of-color flashes.
Play-of-color, intensity, and pattern are important value factors.
Opal hues can range across the spectrum, with a display of a single color, two or three colors, or all the colors of the rainbow. Opal displays background color in addition to play-of-color. Background color, also called body color, is caused by the suspension of tiny impurities within opal’s silica spheres.
Opals are often divided into types based on background color. Some background colors tend to be more prized than others. All other quality factors being equal, many buyers favor the black background color. This is partly because play-of-color tends to stand out attractively against a dark background.
Common terms for play-of-color patterns include:
- Pinfire or pinpoint: Small, closely set patches of color
- Harlequin or mosaic: Broad, angular, closely set patches of color
- Flame: Sweeping reddish bands or streaks that shoot across the stone
- Peacock: Mainly blue and green
No matter the color or combination of colors, play-of-color must be vivid to command a high rating. In other words, opal lovers prize bright play-of-color over faint play-of-color, making play-of-color possibly the most remarkable aspect of an opal’s presence.
Secondary in importance to play-of-color in opal is its color range. If an opal’s play-of-color is not just bright, but also ranges across the entire spectrum, it’s very exceptional and valued. Not every precious opal, however, sparkles with every color of the rainbow. In some, the play-of-color consists of just one main color and two or more secondary colors.
Desirable play-of-color is further broken down by the colors themselves. Traditionally, red is considered the best prominent color, orange the next most desirable, followed by green. However, favored colors can vary with fashion or personal preference.
In addition, an opal’s play-of-color can change along with the viewing angle or type of light. For example, red might dominate in the same portion of an opal cabochon where blue dominates when it’s viewed from a different angle. The most valuable opals display play-of-color from all angles.
Pattern describes the arrangement of an opal’s play-of-color. Like the shapes you see in the clouds, play-of-color takes many forms. In general, specialists prefer large, closely arranged patches of color over tiny, scattered dots. As with any play-of-color, no matter what the pattern, colors must be bright for the stone to be valuable.
In addition to the arrangement and shape of the play-of-color patches, buyers must consider “dead spots”, or extinction when evaluating opal pattern. A dead spot is an area of the gem in which no play-of-color appears and only background color is visible. Dead spots detract from opal value, especially if there are several of them.
Clarity and Transparency
Experts expect different levels of clarity for different types of opals. Clarity in opal is its degree of transparency and freedom from inclusions. An opal’s clarity can range all the way from completely transparent to opaque. A cloudy or milky background color lowers the value of any opal. It makes the gem less attractive, and it can sometimes signal a lack of stability.
There are various types of opal clarity characteristics that affect value. Opals, like other gems, may have inclusions in the form of fractures, or pits and other surface blemishes. An opal might also contain fragments of its host rock, called matrix which usually, but not always, is detrimental to its appearance and value. It depends on the type of opal. Even if cracked opals don’t break right away, they have little durability in jewelry, and the fractures spoil the beauty and clarity of the gem.
If an opal loses moisture, it can lead to crazing: a fine network of cracks that resembles a spider’s web. The moisture loss can be caused by heat or excessive dryness, or by exposure to bright light or direct sunlight.
Fine opals are often cut into irregular shapes that keep as much play-of-color as possible. The cutter considers an opal’s color, pattern, and clarity when planning the finished gem. Opal cutters usually cut top-quality rough to show off its spectacular play-of-color.
On the other hand, cutters typically fashion commercial-quality white opal and assembled opal into standard calibrated sizes, usually ovals.
The cut of a fine opal should be symmetrical. If it’s a cabochon, the dome should be well rounded. Domed surfaces give the best play-of-color, and make the stone appear vivid from most viewing angles. If the cabochon is flat, it might be vulnerable to breakage, especially during setting into jewelry. If it’s too high, it might be hard to set in jewelry.
Opals come in a wide range of sizes and carat weights. Opal has relatively low density compared to many other gemstones so even larger sizes can be comfortable to wear.
Common sizes for many of the opal cabochons set in jewelry are 6×4, 7×5, and 8×6 mm.
Individual opals can vary widely in appearance and quality. As diverse as snowflakes or
fingerprints, each gem can differ noticeably.
There are three main aspects of an opal’s quality:
- Color—Background color and play-of-color
- Pattern—Arrangement of play-of-color
- Clarity—Transparency and quantity of inclusions
Hardness and toughness
Gem and mineral hardness is measured on the Mohs scale. The numbers are based on the relative ease or difficulty with which one mineral can be scratched by another. But the Mohs scale is deceptive. The steps between the minerals are not evenly spaced. For example, diamond is only one number away, but it’s many times harder than gems in the corundum family. Opal hardness is variable depending on its exact composition and formation conditions, and ranges from 5 to 6.5 on the Mohs scale. Its toughness is very poor to fair, making opal a gem that is suitable for jewelry but requires care when wearing so as to not scratch or break the stone.
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. Opal is attacked by hydrofluoric acid and caustic alkaline solutions.
The only safe way to clean opal is with warm, soapy water.
Because opal has the colors of other gems, the Romans thought it was the most precious and powerful of all. The Bedouins believed that opals contained lightning and fell from the sky during thunderstorms. When Australia’s mines began to produce opals commercially in the 1890s, it quickly became the world’s primary source for this October birthstone.
Writers have compared opals to volcanoes, galaxies, and fireworks. Admirers gave extraordinary opals poetic names like Pandora, Light of the World, and Empress.
Many cultures have credited opal with supernatural origins and powers. Arabic legends say it falls from the heavens in flashes of lightning. The ancient Greeks believed opals gave their owners the gift of prophecy and guarded them from disease. Europeans have long considered the gem a symbol of hope, purity, and truth.
Opal is the product of seasonal rains that drenched dry ground in regions such as Australia’s semi-desert “outback.” The showers soaked deep into ancient underground rock, carrying dissolved silica (a compound of silicon and oxygen) downward. During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal.
How Opal Forms
Opal is known for its unique display of flashing rainbow colors called play-of-color. There are two broad classes of opal: precious and common. Precious opal displays play-of-color, common opal does not.
Although experts divide gem opals into many different categories, five of the main types are:
- White or light opal: Translucent to semitranslucent, with play-of-color against a white or light gray background color, called bodycolor.
- Black opal: Translucent to opaque, with play-of-color against a black or other dark background.
- Fire opal: Transparent to translucent, with brown, yellow, orange, or red bodycolor. This material—which often doesn’t show play-of-color—is also known as “Mexican opal.”
- Boulder opal: Translucent to opaque, with play-of-color against a light to dark background. Fragments of the surrounding rock, called matrix, become part of the finished gem.
- Crystal or water opal: Transparent to semitransparent, with a clear background. This type shows exceptional play-of-color.
(Opal, GIA Gem Encyclopedia, 2015)