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Blue Opal in Host Rock refers to occurrences where blue common opal (non-precious opal without play-of-color) is naturally embedded within its original geological matrix. The opal itself is a hydrated amorphous form of silica, characterized by its typically waxy to vitreous luster and a range of blue hues, from pale sky blue to deeper indigo. The host rock can vary significantly, including basalt, rhyolite, sandstone, shale, or even petrified wood, and its characteristics (color, texture, hardness) will influence the overall appearance and stability of the specimen. The blue opal often forms as veins, nodules, or infillings within the host rock.
How to Identify
- Color
- The opal itself exhibits various shades of blue, from light sky blue to grayish-blue or deep indigo. The host rock's color will vary widely depending on its composition (e.g., reddish-brown basalt, light-colored sandstone, dark shale).
- Luster
- Opal typically has a vitreous to waxy or resinous luster. The host rock's luster will depend on its mineralogy (e.g., dull, earthy, glassy).
- Texture
- Opal is typically smooth, conchoidal fracture. The host rock's texture can be granular, aphanitic, porphyritic, or clastic, depending on its type.
- Crystal Form
- Opal is amorphous, meaning it lacks a defined crystal structure. It often forms botryoidal, reniform, stalactitic, or massive habits within the host rock. The host rock will exhibit its characteristic mineral grains or rock fabric.
- Cleavage
- Opal has no cleavage. The host rock's cleavage or fracture will depend on its constituent minerals and rock type.
- Geological Environment
- Found in areas of past or present volcanic activity (e.g., basalt, rhyolite), sedimentary basins where silica-rich waters have percolated through porous rocks (e.g., sandstone, shale), or in weathered zones of various rock types. Often associated with hot springs, geysers, or areas of intense weathering and alteration.
Key Facts
- Hardness: 5.5 - 6.5 (Mohs scale)
- Specific Gravity: 1.9 - 2.3
- Crystal System: Amorphous
- Color: Various shades of blue (sky blue, grayish-blue, indigo) for the opal; host rock color is variable.
- Luster: Vitreous to waxy or resinous (for opal); host rock luster is variable.
- Transparency: Translucent to opaque (for opal)
- Fracture: Conchoidal (for opal)
- Cleavage: None (for opal)
- Composition: Hydrous silicon dioxide (SiO2·nH2O) for opal; host rock composition is variable.
Quick Check
- Color: Blue (various shades) within a contrasting host rock
- Luster: Vitreous to waxy or resinous (for opal)
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, botryoidal, reniform, stalactitic, or as vein fillings within the host rock.
- Cleavage Type: Absent
- Fracture Type: Conchoidal
- Tenacity: Brittle
- Luster Type: Vitreous to waxy or resinous
Formation
Blue opal forms from the precipitation of hydrous silicon dioxide (SiO2·nH2O) from silica-rich waters. This process typically occurs in low-temperature, near-surface environments where silica-rich solutions percolate through porous host rocks, filling voids, fractures, and replacing organic material. The blue color in common opal is often attributed to the scattering of light by microscopic inclusions or structural irregularities within the silica spheres, or sometimes by trace elements like copper or titanium, though the latter is less common for common blue opal. The host rock provides the structural framework and source of silica in many instances, or simply acts as the matrix in which the opal forms.
Usage
Primarily as a collector's specimen, ornamental stone, and for lapidary purposes. While precious opal is highly valued for its play-of-color, common blue opal is appreciated for its aesthetic appeal and often cut into cabochons, beads, or used in carvings. The host rock provides stability and can enhance the aesthetic presentation of the opal.
Age Distribution
Variable, ranging from Cenozoic to Mesozoic, depending on the specific geological setting and host rock formation.
Where to Find
Oregon, USA
Known for blue opal found in volcanic host rocks, particularly rhyolite. Examples include the Opal Butte area.
Nevada, USA
Various localities yield common blue opal in volcanic and sedimentary host rocks.
Indonesia
Some regions produce blue common opal, often associated with volcanic activity and petrified wood.
Peru
Known for its distinctive blue opal, often found in nodular or massive forms within host rock.
Australia
While famous for precious opal, common blue opal also occurs in various sedimentary and weathered environments.
Finding Tips
Look for Volcanic or Sedimentary Environments
Focus on areas with past volcanic activity (rhyolite, basalt flows) or sedimentary basins with evidence of silica-rich water circulation, such as hot spring deposits or weathered zones.
Identify Host Rock Characteristics
Familiarize yourself with the common host rocks for opal in a given region. Look for fractures, vugs, or altered zones within these rocks that might contain opal.
Observe Luster and Fracture
Opal's characteristic waxy to vitreous luster and conchoidal fracture can help distinguish it from surrounding host rock minerals. Look for these features in exposed surfaces.
Check for Translucency
Common blue opal is often translucent to opaque. Hold specimens up to light to check for light transmission, which can help differentiate it from opaque host rock.
Consider Hardness
Opal is relatively soft (5.5-6.5 on Mohs scale). While not a definitive test in the field, it can help distinguish it from harder quartz or chalcedony in some cases.
Similar Rocks
Chrysocolla
CuSiO3·nH2O
Also known as: Gem Silica (when pure and translucent)
Turquoise
CuAl6(PO4)4(OH)8·4H2O
Also known as: Callais
Chalcedony (Blue)
SiO2
Also known as: Blue Lace Agate, Mohave Blue
Scientific Classification
- Mineral Class
- Silicates (specifically, a mineraloid)
- Group
- Opal Group
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Silicon dioxide with varying amounts of water (typically 3-21% by weight)
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