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Blue Opal is a variety of common opal, meaning it typically does not exhibit the 'play-of-color' phenomenon characteristic of precious opal. Instead, it displays a uniform body color, ranging from pale sky blue to a deeper, more intense blue, sometimes with hints of green. It is often translucent to opaque, with a waxy to vitreous luster. The most well-known blue opals come from Peru, where they are often found in nodular or botryoidal forms within volcanic rocks. Its serene blue hue makes it a popular choice for jewelry and decorative items.
How to Identify
- Color
- Ranges from pale sky blue to medium or deep blue, sometimes with greenish tints. The color is generally uniform throughout the stone, without the iridescent play-of-color seen in precious opal.
- Luster
- Waxy to vitreous (glassy) when polished, dull to earthy in rough specimens.
- Texture
- Smooth to slightly conchoidal fracture surfaces. Can be found in botryoidal (grape-like) or reniform (kidney-shaped) masses.
- Crystal Form
- Amorphous (lacks a defined crystal structure). Occurs as massive, botryoidal, reniform, or stalactitic aggregates, filling veins, nodules, or cavities.
- Cleavage
- None, as it is amorphous.
- Geological Environment
- Typically found in volcanic rocks (e.g., rhyolite, andesite, basalt) where silica-rich waters have percolated through fractures and cavities. Also found in sedimentary environments, particularly in areas with high silica content and hydrothermal activity.
Key Facts
- Hardness: 5.5 - 6.5 on the Mohs scale
- Specific Gravity: 1.98 - 2.25
- Crystal System: Amorphous
- Color: Pale blue to deep blue, sometimes with greenish tints
- Luster: Waxy to vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Hydrated amorphous silicon dioxide (SiO2·nH2O)
Quick Check
- Color: Pale to deep blue, uniform
- Luster: Waxy to vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, botryoidal, reniform, stalactitic, nodular, vein fillings
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy to vitreous
Formation
Blue opal, like other opals, forms from a solution of silicon dioxide and water. As water seeps through the Earth's crust, it picks up silica from sandstone and other rocks. This silica-rich solution then deposits in cracks and voids, such as those caused by natural faults or decaying organic matter. Over long periods, typically under low temperature and pressure conditions, the water evaporates, leaving behind a silica gel that hardens into opal. The blue color in some opals is often attributed to the presence of microscopic inclusions of other minerals, such as chrysocolla, or to the scattering of light by sub-microscopic spheres of silica, similar to the play-of-color mechanism in precious opal, but resulting in a uniform blue hue due to the specific size and arrangement of these spheres.
Usage
Primarily used as a gemstone for jewelry (cabochons, beads, carvings). Its unique blue color and often translucent to opaque nature make it popular for ornamental purposes. It is generally not used in industrial applications due to its relative softness and preciousness.
Age Distribution
Opal formation is a continuous geological process, but significant deposits often form in Cenozoic volcanic and sedimentary environments. The age of specific blue opal deposits varies widely depending on the geological setting, ranging from millions to tens of millions of years old.
Where to Find
Peru
The most significant source of high-quality blue opal, particularly from the Andes Mountains. Often referred to as Peruvian Blue Opal or Andean Opal. Found in volcanic host rocks.
Oregon, USA
Some blue opal occurrences, often associated with volcanic activity.
Indonesia
Limited occurrences of blue opal, sometimes associated with petrified wood.
Nevada, USA
Certain localities yield common opal with blue hues.
Finding Tips
Look for Volcanic Terrains
Focus your search in areas with past or present volcanic activity, as these environments are conducive to opal formation.
Examine Fractures and Cavities
Opal often forms in voids, cracks, and fissures within host rocks. Look for these features in exposed rock faces or weathered material.
Identify Associated Minerals
Opal can be found alongside other silica minerals like chalcedony or quartz, and sometimes with copper minerals if the blue color is due to chrysocolla inclusions.
Check for Waxy Luster
Even in rough form, opal can exhibit a distinctive waxy to resinous luster, especially on freshly broken surfaces.
Consider Weathering
Weathering can expose opal deposits. Look for areas where host rocks are eroding, potentially revealing opal nodules or veins.
Similar Rocks
Chalcedony (Blue)
Chalcedony (variety: Blue)
Also known as: Blue Lace Agate, Sapphire Chalcedony
Turquoise
Turquoise
Also known as: Callais
Chrysocolla
Chrysocolla
Also known as: Gem Silica
Larimar
Pectolite (variety: Larimar)
Also known as: Stefilia's Stone
Scientific Classification
- Mineral Class
- Mineraloid
- Group
- Silica Group
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Hydrated amorphous silicon dioxide
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