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Opal is a mineraloid, not a true mineral, because it lacks a crystalline structure (it is amorphous) and has a variable chemical composition, specifically its water content. It is composed of hydrated amorphous silicon dioxide (SiO2·nH2O). The most prized variety, precious opal, exhibits a phenomenon called 'play-of-color,' a kaleidoscopic display of spectral hues that shifts with the angle of observation. This optical effect is caused by the diffraction of light by regularly arranged, sub-microscopic silica spheres within the opal's structure. Common opal, or 'potch,' does not display play-of-color and is typically opaque to translucent with a duller appearance. Opal can occur in a wide range of colors, including white, black, gray, yellow, orange, green, blue, and pink, often with a waxy or vitreous luster.
How to Identify
- Color
- Extremely variable. Precious opal exhibits a 'play-of-color' with iridescent flashes of red, orange, yellow, green, blue, and violet against a body color that can be white, black, gray, blue, green, or orange. Common opal can be white, gray, blue, green, pink, brown, or colorless, without play-of-color.
- Luster
- Vitreous (glassy) to waxy or resinous.
- Texture
- Smooth, often conchoidal fracture surfaces. Can be botryoidal, reniform, or stalactitic in habit.
- Crystal Form
- Amorphous; does not form true crystals. Occurs as botryoidal, reniform, stalactitic, or massive aggregates, or as vein fillings and replacements of organic material (e.g., wood, shells).
- Cleavage
- None. Opal is amorphous and lacks a defined crystal lattice, thus it does not exhibit cleavage.
- Geological Environment
- Opal forms in sedimentary and volcanic environments. In sedimentary settings, it typically occurs in arid or semi-arid regions where silica-rich groundwater percolates through porous rocks like sandstone, mudstone, or diatomaceous earth. It can fill fissures, cavities, or replace organic material. In volcanic settings, it forms in hot spring deposits (geyserite) or as amygdule fillings in volcanic rocks where silica-rich hydrothermal fluids are present. It is often found in association with limonite, sandstone, rhyolite, and basalt.
Key Facts
- Hardness: 5.5 - 6.5 on the Mohs scale. Relatively soft for a gemstone, making it susceptible to scratching.
- Specific Gravity: 1.98 - 2.25. Varies with water content; lower specific gravity indicates higher water content.
- Crystal System: Amorphous (lacks a crystal system).
- Color: Extremely varied, including white, black, gray, blue, green, yellow, orange, red, pink, and colorless. Precious opal exhibits play-of-color.
- Luster: Vitreous to waxy or resinous.
- Transparency: Transparent to opaque.
- Fracture: Conchoidal (shell-like) to uneven.
- Cleavage: None.
- Composition: Hydrated amorphous silicon dioxide (SiO2·nH2O).
Quick Check
- Color: Highly variable, from colorless to white, black, gray, red, orange, yellow, green, blue, pink. Precious opal shows 'play-of-color'.
- Luster: Vitreous to waxy or resinous.
- 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.
- Tenacity: Brittle.
- Luster Type: Vitreous, waxy, or resinous.
Formation
Opal forms from the precipitation of silica-rich solutions at low temperatures, typically below 100°C. These solutions, often derived from the weathering of silica-bearing rocks (like sandstone, chert, or volcanic ash), percolate through porous host rocks. As the water evaporates or is absorbed, microscopic spheres of amorphous silica (SiO2) are deposited. In precious opal, these silica spheres are uniform in size and arranged in a regular, three-dimensional array, creating a diffraction grating that splits light into its spectral colors (play-of-color). In common opal (potch), the silica spheres are irregularly sized or arranged, preventing play-of-color. The water content (nH2O) can vary significantly, typically ranging from 3% to 21% by weight, but usually between 6% and 10%.
Usage
Precious opal is primarily used as a gemstone in jewelry due to its unique play-of-color. Common opal, lacking play-of-color, has limited industrial uses but can be used as an ornamental stone or for carvings. Diatomaceous earth, a form of biogenic opal, is used as a filter aid, abrasive, and insecticide. Synthetic opals are produced for jewelry and optical applications.
Age Distribution
Opal formation can occur over a wide range of geological ages, from relatively recent (e.g., Cenozoic) to much older deposits (e.g., Mesozoic). The age is dependent on the specific geological environment and the timing of silica-rich fluid circulation.
Where to Find
Australia
The world's primary source of precious opal, particularly black opal (Lightning Ridge, New South Wales), white opal (Coober Pedy, South Australia; Mintabie, South Australia; Andamooka, South Australia), and boulder opal (Queensland). These deposits are typically found in Cretaceous sedimentary basins.
Ethiopia
A significant source of Welo opal (hydrophane opal) and other precious opals, often found in volcanic rocks (rhyolites) in the Wollo Province. These deposits are relatively recent discoveries but have become major producers.
Mexico
Known for its vibrant fire opals, which are typically orange, red, or yellow, often without play-of-color, but some exhibit it. These opals are found in volcanic rocks, particularly rhyolites, in states like Querétaro, Jalisco, and Hidalgo.
Brazil
Produces common opal and some precious opal, often found in sedimentary formations.
United States
Various localities produce common opal and some precious opal, including Nevada (Virgin Valley black opal in volcanic ash), Idaho, Oregon, and California. Oregon is known for its common opal and some precious opal in volcanic settings.
Hungary (Slovakia)
Historically a significant source of precious opal, particularly from the Dubník mines, which were active for centuries. These deposits are associated with volcanic activity.
Finding Tips
Look for Silica-Rich Environments
Focus on areas with evidence of past or present silica-rich groundwater activity, such as weathered volcanic rocks (rhyolite, basalt), sedimentary basins with sandstone or diatomaceous earth, or hot spring deposits.
Examine Host Rocks
Opal often forms in veins, nodules, or as replacements within host rocks. Look for cracks, fissures, or cavities in sandstone, ironstone (boulder opal), or volcanic ash beds.
Observe for Play-of-Color
When searching for precious opal, rotate potential specimens under natural light to detect the characteristic play-of-color. Even small flashes can indicate the presence of precious opal.
Check for Botryoidal or Reniform Habits
Common opal often forms in rounded, grape-like (botryoidal) or kidney-shaped (reniform) masses, which can be indicators of opalization.
Consider Hydration
Opal's water content can make it susceptible to 'crazing' (cracking) if it dries out too quickly. Handle specimens carefully and consider storing them in a humid environment if they appear to be hydrophane (water-absorbing) or freshly collected.
Similar Rocks
Chalcedony
Cryptocrystalline quartz (SiO2)
Also known as: Agate, Jasper, Carnelian, Onyx
Quartz
Silicon dioxide (SiO2)
Also known as: Amethyst, Citrine, Rose Quartz, Smoky Quartz
Glass
Amorphous silicon dioxide (SiO2) with various modifiers
Also known as: Obsidian (natural glass), Man-made glass
Scientific Classification
- Mineral Class
- Mineraloid (Silica group)
- Group
- Silica group
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Hydrated amorphous silicon dioxide, with variable water content (typically 3-21% by weight).
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