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Introduction:
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The October birthstone, opal, occurs in a variety of colors. The play-of-color commonly associated with opals is attributed to tiny spheres of crystobalite layered in siliceous jelly that cause reflection and the inference of patterns. Opal is a very sensitive stone. It breaks and cracks easily when knocked or put under pressure. It is also reactive to chemicals. It is important to take care when working with opals because a small amount of heat can crack the stone. Some popular opal colors include black opal--with a dark gray, dark blue, dark green or gray-black body color--and white opals--with white or other light body colors, which demonstrate play-of-color and internal fire. "Mexican" fire opal--with a yellow, orange or red body color may or may not exhibit play-of-color. Transparent stones with no play-of-color are usually faceted. Stones with play-of-color are usually cut en cabochon and are more rare and valuable. |
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History
of...
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Until the 19th century, Chekoslovakia was the best source for opal. Australian deposits were discovered next. Some famous deposits in New South Wales are Lightning Ridge and White Cliffs. Southern Australia boasts Coober Pedy and Andamooka. Perhaps one of the most interesting stories about opals revolves around Sir Walter Scott's bestseller, "Anne of Geierstein." In the story, a falsly accused deamoness dies shortly after a drop of holy water falls onto her opal and destroys its fiery color. The easily impressionable readers of the day took the instance as a warning that opals brought bad luck. The opal market suffered a tremendous recession for nearly 50 years! Today, however, consumers may prove less superstitious. Since the Lightning Ridge discovery of 1877, Sir Walter Scott's curse on the opal market has subsided. In its place is a marketplace of consumers looking for unique and beautiful jewelry. Opal does an excellent job filling that niche.
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Technical
Specifications:
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| DESCRIPTION |
| Opal is a mineral species. |
| Chemical
Composition: |
SiO2·nH2O (water content varies from 3 to 10%, but can be as high as 20%) |
| Appearance: |
•Transparent to opaque
•Virtually any body color
•Displays several phenomena: play-of-color, asterism (rare), chatoyancy (rare) |
| Misnomers: |
Muller's glass (refers to hyalite) |
| Typical
Sizes: |
Large pieces are available |
| Cut Styles: |
Cabochons, carvings, faceted, beads |
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| IDENTIFICATION |
| Optic
Character: |
SR, ADR common due to strain |
| Refractive
Index: |
1.450 (+.020, -.080); Mexican opal may read as low as 1.37, but it is generally 1.42 to 1.43 |
| Birefingence: |
None |
| Dispersion: |
None Given |
| Pleochroism: |
None |
| Ultraviolet
Fluorescence: |
• Black or white body color: inert to white to moderate light blue, green or yellow (LW and SW); may phosphoresce
• Common opal: inert to strong green or yellowish green (LW and SW); may phosphoresce
•Fire opal: inert to moderate greenish brown (LW and SW); may phosphoresce |
| Absorption
Spectra: |
Green--660 nm, 470 nm cut-off |
| Cause
of Color: |
•Play-of-color: diffraction effects from tiny regularly stacked silica spheres
•Green: nickel in clay impurities |
| Specific
Gravity: |
2.15 (+.08, -.90) |
| Polish
Luster: |
Vitreous to resinous |
| Fracture: |
Conchoidal to uneven |
| Luster: |
Subvitreous to waxy (the latter is more typical of common opal) |
| Cleavage: |
None |
| Indentifying
Characteristics: |
Play-of-color |
| Known
Enhancements: |
• Stones are commonly impregnated with oil, wax or plastic to improve play-of-color and disguise cracks. Oil and wax are not stable treatments, but the plastic enhancements, which require special laboratory equipment, are excellent. A less common use of plastic impregnation is to use black plastic to create the illusion of black opal. To detect the black plastic, look under magnification for concentration of color in cracks. Another sign is that the SG will be too low for gem black opal.
•Another common treatment used to simulate black opal involves the application on aniline dye, silver nitrate, or sugar carbonized by acid. The treatment offers only good to poor stability because the dyes may fade. It can be detected under magnification by looking for concentrated color in cracks.
• Smoke impregnation is commonly used to create the apperance of black opal. This treatment offers only good to poor stability because there is generally shallow penetration. The treament can be detected because the stone will loose its play-of-color when wet. In some cases the stone may even float until it fills with enough water to sink.
•Opal is seldomly backed with reflective foil or black paint to improve play-of-color or simulate black opal. The treatment is fair to poor in stability and can be detected with a carefully applied needle probe "scratch" test. |
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| DURABILITY RATINGS |
| Hardness: |
5 to 6 1/2 |
| Toughness: |
Very poor to fair |
| Opal is sometimes mistaken for: ammolite, mother-of-pearl, labradorite and moonstone |
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Care
Tips:
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| Cleaning Methods |
| •Ultrasonic: |
Never |
| •Steamer: |
Never |
| •Warm
soapy water: |
Safe |
| Stability of Stone |
| •Heat
tolerance: |
Sudden changes in temperature may cause opals to crack, craze, or fracture; overheating will turn most opals white or brownish, and the play-of-color will disappear |
| •Light
tolerance: |
Stable |
| •Chemical tolerance: |
Attacked by hydrofluoric acid and caustic alkalies |
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Recommended
Uses:
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Rings
- Although an opal ring makes for a beautiful piece of jewelry it is not a recommended use for the stone. Unfortunately opal is very delicate and quite susceptible to abuse. Rings, by nature, take more abuse than do other pieces of jewelry and an opal will not tolerate to much banging around. For the careful and loving wearer, there is no substitute for an opal full of fire and color. Although not recommended, someone willing to take the risk and provide TLC to an opal ring would get many years of pleasure and have a beautifully unique piece of jewelry. Earrings
- Opal earrings make for a great addition to any jewelry collection. Worn casually or formally, opal earrings will bring life to your fashion statement. Because earrings do not withstand much abuse opals are suited for the application. Readily available in matching sets, depending on size, opals can be easy to acquire and are quite affordable. Larger size opals are available but may be costly and difficult to find in matching sets.
Pendants - A pendant is the safest use for opal. Specifically, larger sized pendants make for a great finished piece. A pendant provides an opportunity to show off the unique nature of opal and should be considered the best option when thinking of opal jewelry.
Bracelet - Based on durability of the stone, opal is not the best choice for a bracelet setting. However, if great care is taken, an opal bracelet makes for quite a unique piece of occassional wear jewelry. Opals are readily available in matching sets and larger pieces and are economically a good choice for a bracelet, but wearers should be cautious.
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Source
Map:
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| All technical specifications and care
tips on this page was referrenced from the "Gem
Reference Guide" published by the Gemological Institute
of America |
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