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Precious Opal is a hydrated amorphous form of silica, meaning it does not have a crystalline structure like most minerals. Its defining characteristic is 'play-of-color,' an optical phenomenon where spectral colors (red, orange, yellow, green, blue, indigo, violet) flash and shift as the opal is moved or as the light source changes. This phenomenon is caused by the diffraction of light by an ordered internal structure of silica spheres. The body color of precious opal can vary widely, including white, black, gray, yellow, orange, and colorless (crystal opal).
How to Identify
- Color
- Exhibits 'play-of-color' – flashes of spectral colors (red, orange, yellow, green, blue, violet) that shift with angle of observation. Body color can be white, black, gray, blue, green, yellow, orange, or colorless.
- Luster
- Vitreous to waxy or resinous.
- Texture
- Smooth, conchoidal fracture surfaces.
- Crystal Form
- Amorphous; typically occurs as botryoidal, reniform, stalactitic, or massive aggregates, or as vein fillings and nodules. Does not form distinct crystals.
- Cleavage
- None.
- Geological Environment
- Found in sedimentary rocks (especially sandstone and claystone), volcanic rocks (rhyolite, basalt), and as a precipitate from hot springs. Often fills veins, cavities, and replaces organic material (e.g., wood, shells).
Key Facts
- Hardness: 5.5-6.5 on Mohs scale.
- Specific Gravity: 1.98-2.25 (variable due to water content).
- Crystal System: Amorphous (no crystal system).
- Color: Variable body colors (white, black, gray, blue, green, yellow, orange, colorless) with characteristic 'play-of-color'.
- Luster: Vitreous to waxy or resinous.
- Transparency: Transparent to opaque.
- Fracture: Conchoidal (shell-like).
- Cleavage: None.
- Composition: Hydrated amorphous silicon dioxide (SiO2·nH2O).
Quick Check
- Color: Play-of-color (flashes of spectral colors); various body colors.
- Luster: Vitreous to waxy/resinous.
- Streak: White.
Physical Characteristics
- Crystal Habit: Massive, botryoidal, reniform, stalactitic, nodular, or as vein fillings. Never forms euhedral crystals.
- Cleavage Type: None.
- Fracture Type: Conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to waxy or resinous.
Formation
Precious Opal forms from a silica-rich solution that percolates through cracks and voids in rocks. As the water evaporates, microscopic spheres of silica (amorphous silicon dioxide) are deposited. The 'play-of-color' characteristic of precious opal arises from the diffraction of light by these uniformly sized and ordered silica spheres. The spheres must be of a specific size (typically 150-350 nanometers) and arranged in a regular, three-dimensional array to produce the spectral colors. The water content (nH2O) can range from 3% to 21% by weight, typically 6-10% in precious opal. This process is often associated with weathering of silica-rich rocks, volcanic activity, or deposition in sedimentary basins.
Usage
Precious Opal is primarily used as a gemstone in jewelry due to its unique play-of-color. It is cut and polished into cabochons, beads, and carvings. Its value is determined by the intensity, pattern, and range of colors displayed, as well as its body tone (black, dark, light, or crystal opal).
Age Distribution
Opal formation can occur over various geological timescales, from relatively recent (millions of years) to much older deposits. The age of specific opal deposits varies significantly, with some Australian deposits dating back to the Cretaceous period (approximately 100 million years ago).
Where to Find
Australia
The world's primary source of precious opal, particularly from South Australia (Coober Pedy, Mintabie, Andamooka), New South Wales (Lightning Ridge - famous for black opal), and Queensland (boulder opal).
Ethiopia
Significant deposits of Welo opal (hydrophane opal) have emerged, known for their vibrant play-of-color and often found in volcanic rocks.
Mexico
Known for fire opal (with a fiery orange to red body color, sometimes with play-of-color) and water opal, often found in rhyolitic volcanic rocks.
Brazil
Produces some precious opal, often found in sedimentary formations.
USA
Limited deposits in Nevada (Virgin Valley - black opal, often unstable), Idaho, and Oregon.
Finding Tips
Look for specific geological environments
Search in areas known for sedimentary basins with silica-rich solutions, weathered volcanic rocks (rhyolite, basalt), or hot spring deposits. In Australia, look for 'opal dirt' (weathered claystone or sandstone) and 'nobbies' (rounded opal concretions).
Observe light interaction
Precious opal's defining feature is its play-of-color. Rotate suspected specimens under a strong light source to look for flashes of spectral colors. Common opal (potch) will not exhibit this phenomenon.
Check for specific gravity and hardness
Opal is relatively light (SG 1.98-2.25) and soft (Mohs 5.5-6.5). It can be scratched by quartz. These properties can help differentiate it from harder, denser minerals.
Examine fracture patterns
Opal exhibits a conchoidal (shell-like) fracture, which is characteristic of amorphous materials.
Be aware of stability
Some opals, particularly those from volcanic environments (e.g., Virgin Valley, Nevada), can be unstable and prone to 'crazing' (cracking) due to dehydration. Hydrophane opals (e.g., Welo opal) can absorb water and change transparency and play-of-color.
Similar Rocks
Common Opal
Opal (SiO2·nH2O)
Also known as: Potch Opal
Chalcedony
SiO2
Also known as: Cryptocrystalline Quartz
Glass
Amorphous SiO2 and other oxides
Also known as: Man-made glass
Scientific Classification
- Mineral Class
- Mineraloid (not a true mineral due to amorphous structure)
- Group
- Silica group (hydrated amorphous silica)
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Silicon dioxide with varying amounts of water (typically 3-21% by weight)
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