Raw opal is a beautiful and valuable gemstone, but it can be difficult to identify. Here are a few tips to help you identify raw opal:
First, look at the color. Raw opal can be found in a variety of colors, including white, black, blue, green, and red. The most common color is white or cream. Raw opal is also known for its play-of-color, which is a unique optical phenomenon that causes the stone to appear to change color as it is moved. Another telltale sign of genuine raw opal is its lack of transparency. Raw opal is usually opaque or translucent, meaning that light cannot pass through it easily. It is the crystalline structure of raw opal that refracts light and produces the play-of-color. The play-of-color is the defining characteristic of raw opal and distinguishes it from other materials.
Finally, feel the weight of the stone. Raw opal is a relatively heavy stone, so it should feel substantial in your hand. If the stone is lightweight, it is likely not real opal. To differentiate the difference between genuine opals and imitations, use a metal file to scratch an inconspicuous area of the stone. Genuine opal will not scratch, whereas an imitation will. Finally, genuine opal, when submerged in water, will appear more transparent.
Characteristics of Raw Opal
Raw opal possesses a captivating play of colors known as opalescence. This mesmerizing effect originates from the unique microstructure of the stone.
Opal is composed of tiny, spherical silica particles arranged in a regular lattice structure. When light waves strike these particles, they undergo diffraction, scattering in different directions. The colors observed depend on the size and organization of these silica spheres, as well as the angle at which the light strikes the stone.
1. Appearance and Texture
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Raw opal typically exhibits a milky or cloudy appearance, known as ‘potch’. This is due to the presence of air bubbles or other inclusions within the stone.
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The surface of raw opal can be smooth or rough, depending on the formation environment. Some opals may have a pitted or porous texture due to the dissolution of silica during crystallization.
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The hardness of raw opal varies from 5.5 to 6.5 on the Mohs scale. It is relatively soft compared to other gemstones and can be easily scratched or damaged.
2. Internal Structure
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Raw opal’s internal structure is characterized by irregular bands or patches of color. These areas represent regions where silica deposition has occurred under different conditions, resulting in variations in the size and arrangement of silica spheres.
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The play of color in raw opal is often concentrated within these internal bands or patches. The movement of light through the stone creates a dynamic and captivating effect.
Physical Properties to Examine
Color
Opal occurs in a wide range of colors, including white, black, red, yellow, green, blue, and purple. The most valuable opals are those that display a play-of-color, an optical phenomenon that causes the stone to appear to change color as it is moved. The play-of-color is caused by the diffraction of light by the silica spheres within the opal.
Transparency
Raw opal is typically opaque, but some varieties may be translucent or even transparent. The transparency of an opal is determined by the size and spacing of the silica spheres. Opals with larger, more closely spaced silica spheres will be more opaque, while opals with smaller, more widely spaced silica spheres will be more transparent.
Hardness
Opal is a relatively soft mineral, with a hardness of 5.5 to 6.5 on the Mohs scale. This means that it can be easily scratched by harder materials, such as quartz or steel. The hardness of opal is an important factor to consider when choosing jewelry settings, as it will determine how well the opal will resist damage.
Specific Gravity
The specific gravity of raw opal is typically between 1.9 and 2.5. This means that opal is lighter than water, but not as light as some other gemstones, such as diamond or sapphire. The specific gravity of opal can be used to identify it from other gemstones.
Table 1: Physical Properties of Raw Opal
| Property | Value |
|---|---|
| Color | White, black, red, yellow, green, blue, purple |
| Transparency | Opaque, translucent, transparent |
| Hardness | 5.5 to 6.5 on the Mohs scale |
| Specific Gravity | 1.9 to 2.5 |
Color and Pattern Recognition
Raw opal displays a remarkable array of colors and patterns. The play of color is one of the most distinctive features of opal, which can exhibit a range of hues, including fiery reds, vibrant greens, deep blues, and shimmering golds. These iridescent hues arise from the interference of light within the opal’s layered structure and give the gemstone its characteristic opalescence.
Identifying Opal by Color
The color of opal can vary significantly, from transparent to opaque. Transparent opals transmit light, while opaque opals block it. Translucent opals allow some light to pass through, resulting in a milky or hazy appearance. When it comes to raw opal, it is essential to examine it under natural light, as the true colors of the opal may not be fully revealed under artificial light.
Interpreting Opal Patterns
Opal can exhibit a wide variety of patterns, including:
- Fire: A vibrant pattern resembling flames
- Harlequin: A bold checkerboard or mosaic-like pattern
- Pinfire: Tiny, sparkling points of color
- Rolling Flash: A shimmering, wave-like pattern
- Jellyfish: A free-form pattern with wispy, flowing lines
These patterns are influenced by the way the silica spheres are arranged within the opal’s structure and can provide insights into the formation and composition of the gemstone.
Distinguishing Raw Opal from Imitations
There are several ways to distinguish raw opal from imitations:
| Characteristic | Genuine Opal | Imitation |
|---|---|---|
| Color | Natural, iridescent play of colors | Often too bright or consistent |
| Pattern | Unique, complex patterns | Regular, artificial patterns |
| Structure | Amorphous, layered structure | Crystalline, regular structure |
By carefully examining the color, pattern, and structure of a raw opal, you can increase your chances of identifying genuine gemstones and avoiding imitations.
Fire and Play-of-Color
The most striking characteristic of raw opal is its unique fire and play-of-color. This phenomenon occurs when light interacts with the opal’s silica spheres, diffracting and refracting to produce a vibrant display of colors. The intensity and brilliance of the fire depend on the size, shape, and orientation of the silica spheres within the opal.
The play-of-color, also known as iridescence, is caused by the interference of light waves as they pass through the opal’s crystal lattice. The specific colors that appear depend on the wavelength of the light and the orientation of the crystal lattice. The most common colors found in opal include red, orange, yellow, green, blue, purple, and pink.
The fire and play-of-color in opal can vary widely in intensity and complexity. Some opals may exhibit a subtle glimmer while others display a vibrant, kaleidoscopic display of colors. The most valuable opals are those that showcase a strong and diverse range of colors and patterns.
| Light Source | Effect on Fire |
|---|---|
| Sunlight | Strongest fire, most intense colors |
| Artificial light | Weaker fire, less vibrant colors |
| Incandescent light | Warmer, more yellow undertones |
| Fluorescent light | Cooler, more blue undertones |
Testing Hardness
Mohs hardness scale, a common tool used by gemologists and geologists. Hardness is determined by scratching the mineral with a series of reference minerals, with each mineral having a specific hardness value. Opal falls between 5 and 6.5 on the Mohs scale, meaning that it can be scratched by a steel file but not by a glass plate. To test an opal’s hardness, scratch its surface with a piece of glass or steel. If the surface is scratched, the opal is likely to be genuine.
Luster
Luster refers to the way a mineral’s surface reflects light. Opal typically exhibits a play-of-color, which is caused by the interference of light waves within the mineral’s structure. This play-of-color can range from iridescent to fiery, and it is one of the most distinctive characteristics of opal. To test an opal’s luster, hold it up to a light source and observe the way the light reflects off the surface. Genuine opal should exhibit a bright, shimmering play-of-color.
Additional Tests
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Density
Opal has a density of around 2.1 to 2.2 g/cm3, which is relatively low compared to other minerals. To test an opal’s density, use a hydrostatic balance or weigh the opal and divide its mass by its volume.
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Color
Opal can display a wide range of colors, including white, black, orange, yellow, green, and blue. The color of an opal is determined by the presence of impurities or inclusions, such as iron or manganese. To identify an opal based on its color, compare it to known samples or refer to a color chart.
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Fluorescence
Some opals exhibit fluorescence, which is the emission of light when exposed to ultraviolet radiation. To test an opal’s fluorescence, use a UV lamp and hold the opal in its light. Genuine opal should emit a bright, fluorescent glow.
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Inclusions
Opal is often found with inclusions, such as dendrites, crystals, or other minerals. These inclusions can provide clues to the opal’s formation and origin. To identify an opal based on its inclusions, examine it under a microscope or with a loupe.
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Play-of-color
The play-of-color is a unique characteristic of genuine opal. This effect is caused by the diffraction of light within the opal’s structure. To test an opal’s play-of-color, hold it up to a light source and tilt it around. Genuine opal should display a bright, shimmering play-of-color that moves as the opal is tilted.
Identifying Matrix and Inclusions
The Matrix:
The matrix of an opal refers to the base rock in which it is found. Common matrix materials include:
| Matrix Type | Appearance |
|---|---|
| Sandstone | Grainy or porous, typically yellow or brown |
| Ironstone | Dark brown to reddish, often with a metallic sheen |
| Basalt | Black or dark gray, fine-grained and dense |
| Rhyolite | Light-colored and glassy, with small crystals |
Inclusions:
Inclusions are foreign materials found within the opal. They can provide clues to the opal’s origin and environment of formation:
| Inclusion Type | Appearance and Significance |
|---|---|
| Fossils | Plant or animal remains, indicating the age and depositional environment of the opal |
| Minerals | Crystals or mineral matrix, such as quartz, pyrite, or garnets, providing information about the geological processes involved in opal formation |
| Bubbles | Air or gas bubbles trapped within the opal, indicating fluid movement and pressure changes during opalization |
| Dendrites | Tree-like or branching patterns caused by iron or manganese oxide, indicating precipitation from groundwater |
| Fire | Red or orange patches within the opal, caused by the oxidation of iron impurities |
Distinguishing from Similar Stones
Identifying raw opal can be challenging due to its resemblance to other gemstones. Here’s how to distinguish raw opal from similar stones:
Moonstone
Moonstone exhibits a milky, iridescent glow called adularescence, which can be mistaken for opal’s play-of-color. However, opal’s play-of-color is more intense and varied, often displaying a wider range of hues.
Labradorite
Labradorite also displays iridescent hues, but they are typically restricted to shades of blue, green, and gold. Additionally, labradorite’s play-of-color is less pronounced and tends to shimmer in one direction, unlike opal’s multi-directional play.
Chalcedony
Chalcedony can mimic opal’s milky appearance but lacks play-of-color. It is typically semi-transparent or translucent, while raw opal is often opaque.
Agate
Agate, a variety of chalcedony, often forms banded or layered patterns that can resemble opal’s play-of-color. However, agate’s patterns are typically consistent throughout the stone, unlike opal’s random and irregular play.
Fire Agate
Fire agate displays a play-of-color similar to opal, but the hues are typically limited to orange, red, and yellow. Additionally, fire agate’s play-of-color is often more transparent and less intense than opal’s.
Rainbow Fluorite
Rainbow fluorite, as its name suggests, exhibits a wide range of colors, but it lacks opal’s play-of-color effect. Instead, fluorite’s colors are distributed throughout the stone as distinct bands or patches.
Synthetic Opal
Synthetic opals, often referred to as ‘Gilson opals,’ mimic the appearance of natural opal, but they are typically brighter and have a more uniform play-of-color. Additionally, synthetic opals are often cut and polished, while raw opal is found in its natural form.
Using a Magnifying Glass
Examine the opal’s surface using a 10x magnifying glass. Raw opal typically exhibits a play of color that changes as the angle of the light changes. Look for flashes of iridescent hues, such as red, green, blue, or purple.
Inspect the opal for any inclusions or imperfections. Raw opal often contains inclusions of other minerals, such as iron oxide or quartz. These inclusions can take the form of streaks, spots, or veins within the opal.
Examine the opal’s luster. Raw opal has a greasy or waxy luster, unlike polished opal, which exhibits a vitreous or glassy luster. This luster is due to the presence of microscopic cavities within the opal’s structure.
Observe the opal’s translucency. Raw opal is typically translucent, allowing light to pass through it to some extent. Hold the opal up to a light source to check its translucency.
Check for fire or play-of-color. When viewed in natural light, raw opal should exhibit a play-of-color, which is the iridescent display of colors that change as the opal is moved or rotated. This play-of-color is due to the refraction of light within the opal’s internal structure.
Assess the weight of the opal. Raw opal is denser than polished opal due to the presence of inclusions and impurities. Hold the opal in your hand to gauge its weight.
Examine the opal’s surface for any cracks or fractures. Raw opal may have surface imperfections or cracks due to extraction or natural weathering. Inspect the opal carefully for any signs of damage.
Compare the opal to known raw opal samples or consult with a gemologist for verification. Bringing the opal to a professional gemologist can provide an accurate identification and assessment of its authenticity.
Consulting with Gemological Experts
Seeking guidance from gemological experts is a valuable step in identifying raw opal. These professionals possess extensive knowledge and experience in identifying gemstones and can provide reliable assessments. Here are a few key aspects to consider:
1. Credibility and Reputation
Choose gemological experts with a proven track record and reputation. Look for organizations accredited by reputable institutions like the Gemological Institute of America (GIA) or the American Gem Society (AGS).
2. Gemological Equipment
Ensure that the gemological experts have access to advanced equipment, such as microscopes, spectrometers, and refractometers. These tools aid in accurately identifying gemstones and distinguishing them from imitations.
3. Sample Submission
Submit a small sample of the raw material to the gemological experts for analysis. This allows them to conduct thorough examinations and provide a definitive identification.
4. Certificate of Authenticity
After identifying the raw material as opal, request a certificate of authenticity from the gemological experts. This certificate documents the gemstone’s characteristics, value, and authenticity, serving as a valuable reference in the future.
Field Identification Techniques
1. Visual Inspection
Raw opal exhibits a unique play-of-color, which refers to the iridescent shimmering effect it displays when light strikes it. The presence of this play-of-color is a key indicator of raw opal, but its intensity and hues can vary depending on the specific type of opal.
2. Texture
Raw opal typically has a smooth, vitreous surface that resembles porcelain or glass. It is not porous and lacks the rough texture of many other gemstones.
3. Hardness
Raw opal is relatively soft, with a Mohs hardness of 5.5 to 6.5. This means it can be easily scratched by harder materials, such as quartz or steel.
4. Density
Raw opal is a dense mineral, with a specific gravity of around 2.2 to 2.6. This means it feels heavier in the hand than many other gemstones of similar size.
5. Transparency
Raw opal can range from translucent to opaque. Translucent opals allow light to pass through them, while opaque opals do not.
6. Inclusions
Raw opal may contain inclusions, which are impurities or foreign materials that have been trapped within the stone during formation. These inclusions can appear as spots, lines, or other patterns.
7. Fluorescence
Some types of raw opal exhibit fluorescence, which is the ability to glow under ultraviolet light. This fluorescence can range from blue to red-orange, depending on the type of opal.
8. Fracture
Raw opal tends to fracture along conchoidal surfaces, meaning it breaks into curved, shell-like pieces.
9. Refractive Index
The refractive index of raw opal is around 1.45 to 1.55. This means it bends light slightly when it passes through the stone.
10. Specific Gravity
The specific gravity of raw opal is a key identifier. Raw opal typically has a specific gravity of around 1.9 to 2.5, which means it will float in water if it is dropped in. To perform this test, place a small piece of opal in a glass of water. If it floats, it is likely to be raw opal.
How To Identify Raw Opal
Raw opal is a beautiful and valuable gemstone that can be found in a variety of colors. It is a form of silica that is deposited in the ground over time. Raw opal is typically found in the form of nodules, which are small, round rocks. These nodules can be found in a variety of colors, including white, black, blue, green, and red. Raw opal is typically cut and polished before it is used in jewelry. However, it can also be used in its natural state.
There are a few key things to look for when identifying raw opal. First, the stone should be hard. Opal is a 5.5 on the Mohs scale of hardness, which means that it is harder than glass but softer than quartz. Second, the stone should be translucent or opaque. Raw opal is typically not transparent. Third, the stone should have a play of color. This is the most distinctive characteristic of opal. The play of color is caused by the way that light is scattered within the stone. The play of color can vary from opal to opal, and it can be very beautiful.
People Also Ask About How To Identify Raw Opal
What is the difference between raw opal and finished opal?
Raw opal is opal that has not been cut or polished. Finished opal is opal that has been cut and polished to a smooth finish. Raw opal is typically less expensive than finished opal, but it can also be more difficult to find.
How can I tell if raw opal is real?
There are a few ways to tell if raw opal is real. First, the stone should be hard. Opal is a 5.5 on the Mohs scale of hardness, which means that it is harder than glass but softer than quartz. Second, the stone should be translucent or opaque. Raw opal is typically not transparent. Third, the stone should have a play of color. This is the most distinctive characteristic of opal. The play of color can vary from opal to opal, and it can be very beautiful.
How can I clean raw opal?
Raw opal can be cleaned with a soft cloth and warm water. Avoid using harsh chemicals or detergents, as these can damage the stone. You can also use a soft brush to remove any dirt or debris from the stone.
