Raw opal, a captivating gemstone known for its mesmerizing play of colors, is a highly sought-after material for jewelry and art. However, identifying genuine raw opal from imitations or other minerals can be challenging. This comprehensive guide will provide you with valuable insights to help you confidently identify and appreciate this precious stone in its natural state. By exploring its unique characteristics, geological origins, and distinguishing features, you will gain the knowledge necessary to make informed decisions when encountering raw opal.
Raw opal, also known as rough opal, is typically found in the form of nodules or veins within host rocks. These rocks, often sandstone or rhyolite, provide the necessary conditions for opal formation. As silica-rich fluids seep into the host rock, they gradually fill cavities and replace the surrounding minerals. Over time, the silica transforms into opal, exhibiting a distinctive internal structure that gives rise to its extraordinary play of colors. This iridescent phenomenon, known as opalescence, is caused by the diffraction of light within the opal’s microscopic spherical structures. The size, shape, and arrangement of these spheres determine the specific color patterns and brilliance of each opal.
To effectively identify raw opal, several key characteristics must be considered. Firstly, its hardness is a valuable indicator. Raw opal typically ranges in hardness from 5.5 to 6.5 on the Mohs scale, making it relatively soft compared to other gemstones. This softness can be readily tested by gently scratching the surface of the opal with a knife or glass. If the opal scratches easily, it is likely a genuine specimen. Additionally, raw opal exhibits a characteristic conchoidal fracture, meaning that when broken, it tends to form smooth, curved surfaces. This fracture pattern is distinct from the splintery or uneven fractures observed in many other minerals.
Physical Characteristics
Identifying raw opal can be a daunting task, but understanding its unique physical characteristics can make it easier to distinguish it from other gemstones.
Raw opal typically appears as an irregular, rough-surfaced stone. It often exhibits a milky or opaque appearance, with a play of colors or iridescence visible under certain lighting conditions.
Let’s delve into the specific characteristics of raw opal:
Color and Iridescence
| Color | Iridescence |
|---|---|
| Milky or opaque | Play of colors or iridescence |
Raw opal’s unique play of colors, known as iridescence, is its most distinguishing feature.
The iridescence occurs due to the interference of light waves within the opal’s microscopic silica spheres. As light passes through the spheres, it scatters and reflects, producing a rainbow effect.
Various factors, such as the size and arrangement of the silica spheres, influence the specific colors and patterns seen in raw opal. Common hues include blues, greens, reds, and oranges.
Color and Patterns
Raw opal is distinguished by its captivating play of colors, a phenomenon known as opalescence. The gemstone displays a spectrum of iridescent hues that shift and change depending on the viewing angle and light conditions. These colors result from the diffraction of light within the opal’s internal structure. Raw opal is commonly found in shades of blue, green, red, yellow, and orange, creating mesmerising and unique patterns.
Types of Patterns
Raw opal exhibits a wide range of patterns, each adding to its distinct character.
| Pattern | Description |
|---|---|
| Fire Opal | Displays a fiery play of colors with a transparent to semi-transparent body |
| Hydrophane Opal | Appears dull when dry but becomes transparent when wet, revealing its play of colors |
| Matrix Opal | Embeds opal formations within a host rock, creating a unique and earthy appearance |
| Boulder Opal | Formed when opal fills the cavities of a sedimentary rock, resulting in a natural matrix surrounding the opal |
| Dendritic Opal | Showcases patterns resembling tree branches or fern leaves, formed by iron oxide inclusions |
Luster and Transparency
Opal’s luster can vary from vitreous (glassy) to resinous (waxy). The best quality opals have a vitreous luster, which gives them a bright and fiery appearance. The transparency of opal can also vary, from opaque to translucent to transparent. Opaque opals do not allow light to pass through them, while transparent opals allow light to pass through them completely.
Fire
Fire is the term used to describe the play of color in opal. The fire of an opal is caused by the diffraction of light as it passes through the opal’s internal structure. The best quality opals have a strong fire, which means that they display a wide range of colors. The fire of an opal can be influenced by the opal’s orientation, the angle at which the light hits the opal, and the observer’s position.
Color
Opal can display a wide range of colors, including white, black, blue, green, red, orange, and yellow. The most common color of opal is white, but black opal is the most valuable. The color of an opal is caused by the presence of impurities in the opal’s silica matrix. These impurities can include iron, manganese, copper, and cobalt.
| Color | Cause |
|—|—|
| White | No impurities |
| Black | Impurities of carbon or manganese dioxide |
| Blue | Impurities of iron or titanium |
| Green | Impurities of chromium or vanadium |
| Red | Impurities of iron or copper |
| Orange | Impurities of iron or manganese |
| Yellow | Impurities of sulfur or iron |
Hardness and Tenacity
Hardness is a measure of a mineral’s resistance to scratching. It is determined by scratching the mineral with a series of harder minerals and observing the results. Opal has a hardness of 5.5 to 6.5 on the Mohs scale of hardness, which means that it can be scratched by a steel knife but not by a copper penny.
Tenacity is a measure of a mineral’s resistance to breaking. It is determined by attempting to break the mineral with a hammer or other tool. Opal has a tenacity of brittle, which means that it will break easily if it is hit or bent.
Types of Opal
There are two main types of opal: common opal and precious opal. Common opal is opaque and does not show any play of color. Precious opal is transparent or translucent and shows a play of color, which is caused by the diffraction of light within the opal’s structure.
Uses of Opal
Opal is used in a variety of jewelry and decorative applications. Common opal is used in cabochons, beads, and other jewelry pieces. Precious opal is used in fine jewelry and is prized for its unique play of color.
| Property | Value |
|—|—|
| Hardness | 5.5 to 6.5 |
| Tenacity | Brittle |
| Type | Common opal, precious opal |
| Use | Jewelry, decorative applications |
Specific Gravity
The specific gravity of raw opal is typically between 1.9 and 2.2, which is higher than the specific gravity of most other minerals. This means that opal will sink in water, while most other minerals will float. However, it is important to note that the specific gravity of opal can vary depending on its water content. Opal that is more hydrated will have a higher specific gravity than opal that is less hydrated.
The specific gravity of opal can be used to help identify it from other minerals. To test the specific gravity of opal, you can place it in a container of water and see if it sinks or floats. If it sinks, it is likely to be opal. However, if it floats, it is not necessarily not opal. It could be another mineral with a similar specific gravity, such as quartz.
In addition to specific gravity, there are a number of other factors that can be used to help identify raw opal. These factors include its color, luster, and hardness. Opal can be found in a wide range of colors, including white, black, red, orange, yellow, green, and blue. It has a vitreous luster, which means that it looks like glass. Opal is also relatively soft, with a hardness of 5.5 on the Mohs scale.
The following table summarizes the key properties of raw opal:
| Property | Value |
|---|---|
| Specific gravity | 1.9 – 2.2 |
| Color | White, black, red, orange, yellow, green, blue |
| Luster | Vitreous |
| Hardness | 5.5 on the Mohs scale |
Fluorescence
Raw opal exhibits a distinctive fluorescence when exposed to ultraviolet (UV) light. This phenomenon occurs due to the presence of impurities and structural defects within the opal matrix. Different types of opal display variations in their fluorescence properties.
1. Common Fluorescence Colors
The most common fluorescence colors observed in raw opal include:
| Fluorescence Color | Associated Opal Type |
|---|---|
| Red | Coober Pedy boulder opal |
| Green | Ethiopian opal |
| Blue | Mexican fire opal |
| White | Australian white opal |
2. Intensity of Fluorescence
The intensity of fluorescence in raw opal can vary from weak to strong. Factors influencing fluorescence intensity include the concentration and distribution of impurities, as well as the thickness and transparency of the opal.
3. Color Variation within a Gemstone
Some raw opals may exhibit different fluorescence colors within the same gemstone. This phenomenon is known as “play-of-color” and is associated with the presence of multiple impurities and variations in their distribution.
4. Fluorescence in Different Environments
The fluorescence of raw opal can be affected by the surrounding environment. Factors such as the type of UV light source, the distance from the light source, and the viewing angle can influence the perceived fluorescence color and intensity.
5. Fluorescence as an Identification Tool
Fluorescence can be a valuable tool for identifying raw opal. By observing the fluorescence color, intensity, and variations, gemologists can differentiate between different types of opal and detect imitations or treatments.
6. Additional Factors Influencing Fluorescence
In addition to the factors mentioned above, other variables can also impact the fluorescence of raw opal. These include:
- The age of the opal
- The presence of surface treatments or coatings
- The thermal history of the opal
- The presence of other minerals or inclusions
Inclusions and Blemishes
Inclusions and blemishes are natural imperfections that can occur within raw opal. These imperfections can affect the value of the opal, but they can also add to its character and beauty. Some of the most common inclusions and blemishes in opal include:
Cracks
Cracks are one of the most common imperfections found in opal. They can be caused by a variety of factors, such as pressure or heat. Cracks can range in size from microscopic to large and visible cracks. Small cracks may not have a significant impact on the value of the opal, but large cracks can make the opal more fragile and less valuable.
Veins
Veins are another common type of imperfection found in opal. They are caused by the presence of minerals other than opal within the opal. Veins can range in color from white to black. Small veins may not have a significant impact on the value of the opal, but large veins can make the opal less valuable.
Fire Scale
Fire scale is a type of discoloration that can occur in opal. It is caused by the presence of iron oxide within the opal. Fire scale can range in color from yellow to red. Small amounts of fire scale may not have a significant impact on the value of the opal, but large amounts can make the opal less valuable.
Potch
Potch is a type of opal that is not transparent. It is caused by the presence of impurities within the opal. Potch can range in color from white to black. Potch is not as valuable as transparent opal, but it can still be used to create beautiful jewelry and other objects.
| Inclusion or Blemish | Description |
|---|---|
| Cracks | Natural fractures in the opal that can range in size from microscopic to large and visible. |
| Veins | Colored mineral deposits within the opal that can affect its value and appearance. |
| Fire Scale | Discoloration caused by the presence of iron oxide, ranging in color from yellow to red. |
| Potch | Non-transparent opal that is less valuable than transparent opal but still used in jewelry and other objects. |
Examination with Magnification
A hand-lens (also called a loupe) magnifying at least 10x is an indispensable tool for examining raw opal. Use a bright, natural light source, such as sunlight or a daylight lamp, to ensure optimal clarity.
1. Surface Texture
Examine the surface texture of the opal under magnification. Raw opal exhibits a characteristic “play-of-color” that appears as iridescent hues swirling within the stone. The surface may also display pitting or irregularities, indicating natural formation.
2. Internal Structure
Use the hand-lens to peer inside the opal. Look for evidence of layering or banding, which suggests that the opal formed in successive stages. Inclusions, such as small crystals or voids, can also provide clues about the opal’s origin.
3. Light Penetration
Hold the opal up to a light source and observe its transparency. Raw opal typically allows some light to pass through, although its opacity varies depending on its thickness and density.
4. Play-of-Color
Tilt the opal under magnification to observe the “play-of-color.” Note its intensity, pattern, and the range of hues present. These characteristics can help identify the type of opal and its potential value.
5. Inclusions
Examine the opal for inclusions under magnification. Common inclusions in raw opal include small crystals, voids, and dendritic patterns (branching structures). These inclusions can provide insights into the opal’s geological history.
6. Surface Defects
Identify any surface defects, such as scratches, cracks, or pits. These defects may affect the opal’s value, especially if they compromise its structural integrity or play-of-color.
7. Shape and Size
Take note of the opal’s shape and size. Rough opals can vary from small, irregular fragments to large, plate-like specimens. The shape and size of the opal can influence its potential value and applications.
8. Comparison with Known Samples
If possible, compare the raw opal to known samples of identified opal varieties. This can provide a valuable reference point for accurate identification. Consult reputable sources or seek the assistance of a qualified gemologist for further confirmation.
Comparison to Other Gemstones
Color
Raw opal exhibits a wide range of colors, including white, black, blue, green, and yellow. It often displays a unique play-of-color, caused by the interference of light within its structure.
Hardness
Opal has a hardness of 5.5 to 6.5 on the Mohs scale, making it relatively soft and susceptible to scratches.
Transparency
Raw opal can range from transparent to opaque, depending on its water content.
Luster
Opal typically displays a pearly or vitreous luster, with a smooth and shiny surface.
Specific Gravity
The specific gravity of opal ranges from 1.9 to 2.3, making it relatively lightweight.
Crystal Structure
Opal lacks a crystalline structure and is instead an amorphous mineraloid. This means it lacks the ordered atomic arrangement found in true crystals.
Chemical Composition
Opal is primarily composed of silicon dioxide (SiO2), with varying amounts of water. The presence of trace elements can also contribute to its color and other properties.
Heating
Raw opal can lose its water content when heated, resulting in a loss of its play-of-color and becoming opaque and chalky.
Table of Gemstone Comparisons
| Property | Opal | Diamond | Ruby |
|---|---|---|---|
| Color | Wide range | Colorless | Red |
| Hardness | 5.5-6.5 | 10 | 9 |
| Transparency | Transparent to Opaque | Transparent | Transparent |
| Luster | Pearly or Vitreous | Adamantine | Vitreous |
| Specific Gravity | 1.9-2.3 | 3.5 | 4.0 |
| Crystal Structure | Amorphous | Cubic | Trigonal |
| Chemical Composition | SiO2 + H2O | Carbon | Al2O3 with Cr impurities |
| Heating | Loses water and play-of-color | No effect | No effect |
Consulting with Experts
Consulting with experts is a valuable resource for identifying raw opal. Here are some tips:
- Identify local experts through rock and mineral clubs or jewelry associations.
- Attend gemstone shows and workshops where experts can examine your specimens.
- Consult with geologists or mineralogists who have experience with opals.
- Utilize online forums and social media groups to connect with experts.
- Seek advice from gemologists (certified gemstone experts) for professional assessment.
- Use field guides and reference materials to learn about the physical characteristics of different opal varieties.
- Study historical records and mining locations associated with opal discoveries.
- Consider partnering with a reputable opal dealer or collector for guidance and access to raw materials.
- Stay up-to-date with latest research and publications related to opal identification.
- Remember that identifying raw opal can be challenging, but the expertise of experts can provide valuable insights and support.
| Characteristics | Raw Opal |
|---|---|
| Appearance | Rough, earthy texture with varying shades of brown, white, or black |
| Crystalline Structure | Microcrystalline |
| Hardness | 5.5-6.5 on the Mohs scale |
| Density | 1.9-2.2 g/cm³ |
| Refractive Index | 1.44-1.55 |
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 hydrated form of silica, and its unique play of color is caused by the diffraction of light within the stone. Raw opal is often used in jewelry, and it can also be cut and polished into cabochons or other shapes.
There are a few key things to look for when identifying raw opal. First, opal is typically a translucent or opaque stone with a vitreous luster. It is also relatively soft, with a Mohs hardness of 5.5 to 6.5. Raw opal often has a rough or pitted surface, and it may contain inclusions of other minerals.
The most important thing to look for when identifying raw opal is its play of color. Opal’s play of color is caused by the diffraction of light within the stone, and it can be seen in a variety of colors, including blue, green, red, yellow, and orange. The play of color in opal is often referred to as “fire,” and it is what makes this gemstone so unique and valuable.
People Also Ask About How To Identify Raw Opal
How can you tell if opal is real or fake?
There are a few ways to tell if opal is real or fake. First, real opal will have a play of color that is visible from all angles. Fake opal will often only have a play of color that is visible from certain angles.
Another way to tell if opal is real or fake is to look at its surface. Real opal will often have a rough or pitted surface, while fake opal will often have a smooth surface.
What is the difference between raw opal and polished opal?
Raw opal is opal that has not been cut or polished. It is typically a rough or pitted stone with a dull luster. Polished opal is opal that has been cut and polished to a smooth, shiny surface. Polished opal is more valuable than raw opal, and it is often used in jewelry.
How do you care for raw opal?
Raw opal is a relatively delicate gemstone, so it is important to care for it properly. Raw opal should be stored in a cool, dry place, and it should be protected from harsh chemicals and abrasive materials.
