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Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous nature, it is classified as a mineraloid rather than a true mineral. Precious opal exhibits a unique optical phenomenon known as 'play-of-color', which is caused by the diffraction of light by regularly arranged microscopic silica spheres within its structure. Common opal, also known as potch, does not exhibit play-of-color. Opal can occur in a wide range of colors, including colorless, white, gray, black, yellow, orange, red, green, blue, and pink.
How to Identify
- Color
- Extremely variable; colorless, white, gray, black, yellow, orange, red, green, blue, pink. Precious opal exhibits a 'play-of-color' with iridescent flashes of spectral hues.
- Luster
- Vitreous to waxy or resinous.
- Texture
- Smooth, conchoidal fracture surfaces.
- Crystal Form
- Amorphous; typically occurs as botryoidal, reniform, stalactitic, or massive aggregates. Can also fill veins, nodules, or replace organic material (e.g., wood, shells).
- Cleavage
- None, due to its amorphous structure.
- Geological Environment
- Found in sedimentary rocks (especially sandstone), volcanic rocks (e.g., rhyolite, basalt), and as a precipitate from low-temperature hydrothermal solutions. Often associated with areas of ancient hot springs or weathering of silica-rich rocks.
Key Facts
- Hardness: 5.5 - 6.5 on the Mohs scale (relatively soft for a gemstone).
- Specific Gravity: 1.98 - 2.25 (lighter than most gemstones).
- Crystal System: Amorphous (no crystal system).
- Color: Extremely variable; colorless, white, gray, black, yellow, orange, red, green, blue, pink. Precious opal exhibits 'play-of-color'.
- Luster: Vitreous to waxy or resinous.
- Transparency: Transparent to opaque.
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Hydrated silicon dioxide (SiO2·nH2O).
Quick Check
- Color: Variable, often with 'play-of-color' in precious varieties.
- Luster: Vitreous to waxy.
- Streak: White.
Physical Characteristics
- Crystal Habit: Amorphous; typically massive, botryoidal, reniform, stalactitic, or as vein fillings and replacements.
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven, brittle.
- Tenacity: Brittle.
- Luster Type: Vitreous to waxy or resinous.
Formation
Opal forms from a solution of silicon dioxide and water. As water seeps through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids caused by natural faults or decomposing fossils. As the water slowly evaporates, it leaves behind a silica gel that hardens over millions of years to become opal. The presence of microscopic spheres of silica, arranged in a regular pattern, causes the diffraction of light that produces the characteristic 'play-of-color' in precious opal.
Usage
Primarily used as a gemstone due to its unique play-of-color. Common opal is sometimes used for ornamental purposes or as a minor abrasive. Diatomaceous earth, a form of biogenic opal, is used as a filter aid, mild abrasive, and insecticide.
Age Distribution
Can form in various geological ages, from relatively recent to ancient sedimentary and volcanic deposits.
Where to Find
Australia
The world's primary source of precious opal, particularly from localities like Coober Pedy, Lightning Ridge, White Cliffs, and Mintabie. Black opal from Lightning Ridge is particularly famous.
Ethiopia
A significant producer of Welo opal, known for its vibrant play-of-color and hydrophane properties (absorbs water and becomes more transparent).
Mexico
Known for fire opal (orange to red body color, sometimes with play-of-color) and water opal, often found in volcanic rocks.
Brazil
Produces common opal and some precious opal, often found in sedimentary formations.
United States
Localities in Nevada, Idaho, and Oregon produce various types of opal, including precious opal and common opal, often associated with volcanic activity.
Hungary (Slovakia)
Historically a major source of precious opal, particularly from the Dubník mines, though production is now limited.
Finding Tips
Look for specific geological settings
Opal is often found in areas with ancient volcanic activity, sedimentary basins with silica-rich groundwater, or weathered zones of certain rock types. Look for areas with evidence of past hydrothermal activity or silica deposition.
Examine host rocks
In Australia, opal is typically found in Cretaceous sedimentary rocks, often within ironstone concretions or sandstone layers. In volcanic settings, it can be found in vesicles or fractures in rhyolite or basalt.
Check for 'potch' (common opal)
Common opal often occurs alongside precious opal. Finding potch can be an indicator that precious opal might be nearby. Potch can be white, gray, black, or other colors, but lacks the play-of-color.
Use a loupe or magnifying glass
Small flashes of color or subtle iridescence might be visible with magnification, especially when examining rough material.
Wet the specimen
Some opals, particularly hydrophane opals, will show enhanced play-of-color or transparency when wet. This can help in identification, but be aware that some opals can crack if dried too quickly after being saturated.
Safety Precautions
Opal itself is not hazardous. However, mining or collecting in certain geological environments may expose individuals to other hazards such as unstable ground, dust (which can contain crystalline silica), or other minerals. Always wear appropriate personal protective equipment (PPE) and be aware of your surroundings. When cutting or polishing opal, use wet methods to minimize dust inhalation, as fine silica dust can be a respiratory irritant.
Similar Rocks
Chalcedony
SiO2
Also known as: Cryptocrystalline Quartz
Quartz
SiO2
Also known as: Silica
Glass
Amorphous Silicate
Also known as: Volcanic Glass, Obsidian
Scientific Classification
- Mineral Class
- Silicates (specifically, a mineraloid)
- Group
- Silica group (amorphous hydrated silica)
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Silicon dioxide with varying amounts of water (3-21% by weight, typically 6-10%).
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