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Jasper in basalt refers to occurrences where the cryptocrystalline quartz variety, jasper, is found as infillings or veins within a basaltic host rock. Basalt is a fine-grained, mafic extrusive igneous rock, typically dark gray to black. Jasper, a form of chalcedony (microcrystalline quartz), is opaque and can exhibit a wide range of colors (red, yellow, brown, green, blue) due to various mineral inclusions. The contrast between the often dark, fine-grained basalt and the colorful, sometimes patterned jasper creates visually striking specimens.
How to Identify
- Color
- Basalt: Dark gray to black. Jasper: Red, yellow, brown, green, blue, or multicolored, often with distinct patterns or banding. The contrast in color is a key identifier.
- Luster
- Basalt: Dull to sub-vitreous. Jasper: Dull to waxy.
- Texture
- Basalt: Aphanitic (fine-grained) to porphyritic (larger crystals in a fine-grained matrix), sometimes vesicular. Jasper: Smooth, microcrystalline.
- Crystal Form
- Basalt: Typically massive, fine-grained. Jasper: Massive, cryptocrystalline, filling voids or fractures.
- Cleavage
- Basalt: No true cleavage, but can exhibit columnar jointing. Jasper: None.
- Geological Environment
- Volcanic terrains, particularly in areas with basaltic lava flows, flood basalts, or volcanic necks where hydrothermal activity has occurred.
Key Facts
- Hardness: Basalt: 5-6 on Mohs scale. Jasper: 6.5-7 on Mohs scale.
- Specific Gravity: Basalt: 2.8-3.0 g/cm³. Jasper: 2.58-2.91 g/cm³.
- Crystal System: Basalt: Anhedral to subhedral crystals (plagioclase, pyroxene) in a fine-grained matrix. Jasper: Trigonal (cryptocrystalline quartz).
- Color: Basalt: Dark gray to black. Jasper: Red, yellow, brown, green, blue, multicolored.
- Luster: Basalt: Dull to sub-vitreous. Jasper: Dull to waxy.
- Transparency: Basalt: Opaque. Jasper: Opaque.
- Fracture: Basalt: Uneven to conchoidal. Jasper: Conchoidal to splintery.
- Cleavage: Basalt: None (minerals within may have cleavage). Jasper: None.
- Composition: Basalt: Primarily plagioclase feldspar, pyroxene, olivine, and sometimes magnetite. Jasper: Silicon dioxide (SiO2) with various iron oxides/hydroxides and other impurities.
Quick Check
- Color: Dark gray/black basalt with contrasting red, yellow, brown, green, or multicolored opaque inclusions.
- Luster: Dull to sub-vitreous basalt, dull to waxy jasper.
- Streak: Basalt: Gray to black. Jasper: White.
Physical Characteristics
- Crystal Habit: Basalt: Massive, fine-grained, sometimes porphyritic or vesicular. Jasper: Massive, cryptocrystalline, forming nodular, botryoidal, or vein-filling masses.
- Cleavage Type: Basalt: None (minerals within may show cleavage). Jasper: None.
- Fracture Type: Basalt: Uneven to conchoidal. Jasper: Conchoidal to splintery.
- Tenacity: Basalt: Brittle. Jasper: Brittle.
- Luster Type: Basalt: Dull to sub-vitreous. Jasper: Dull to waxy.
Formation
Jasper in basalt forms when silica-rich fluids, often hydrothermal in origin, infiltrate vesicles (gas bubbles) or fractures within basaltic lava flows. The silica precipitates as microcrystalline quartz (jasper), filling these voids. This process can occur during or after the cooling of the basalt.
Usage
Primarily a collector's item due to its aesthetic contrast. Can be used in lapidary arts for cabochons, carvings, and ornamental objects, especially if the jasper is colorful and well-defined within the basalt matrix. Basalt itself is used as aggregate, road base, and in construction.
Age Distribution
Variable, depending on the age of the basaltic host rock and the jasper formation event. Basalts can range from Precambrian to Cenozoic.
Where to Find
Columbia River Basalt Group, USA
Extensive flood basalts in Washington, Oregon, and Idaho, known for various silica infillings including jasper and agate.
Deccan Traps, India
One of the largest volcanic provinces globally, basalts often contain secondary minerals, including jasper.
Iceland
Active volcanic island with abundant basaltic rocks, where hydrothermal alteration can lead to jasper formation.
Australia
Various basaltic regions, particularly in Western Australia and Queensland, can yield jasper in basalt.
Finding Tips
Look for Vesicular Basalts
Jasper often forms by filling vesicles (gas bubbles) in basalt. Look for weathered basalt outcrops where these infillings might be exposed.
Check Fracture Zones
Silica-rich fluids can also precipitate jasper along fractures and faults within basaltic rocks. Examine areas with structural weaknesses.
Hydrothermal Alteration Zones
Areas showing signs of hydrothermal alteration (e.g., discoloration, presence of other secondary minerals) are good indicators for jasper formation.
Examine Weathered Surfaces
Weathering can sometimes expose the jasper more clearly by eroding the softer or more fractured basalt matrix.
Similar Rocks
Agate in Basalt
Agate (SiO2) in Basalt (igneous rock)
Also known as: Basaltic Agate
Chalcedony in Basalt
Chalcedony (SiO2) in Basalt (igneous rock)
Also known as: Basaltic Chalcedony
Opal in Basalt
Opal (SiO2·nH2O) in Basalt (igneous rock)
Also known as: Basaltic Opal
Scientific Classification
- Mineral Class
- Basalt: Igneous rock. Jasper: Silicate (Tectosilicate subclass).
- Group
- Basalt: Mafic extrusive igneous rock. Jasper: Quartz group, Chalcedony variety.
- Crystal System
- Basalt: Predominantly anhedral to subhedral crystals. Jasper: Trigonal (microcrystalline).
- Chemical Formula
- Basalt: Complex silicate mixture (e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene, NaAlSi3O8-CaAl2Si2O8 for plagioclase). Jasper: SiO2.
- Composition
- Basalt: Dominated by plagioclase feldspar and pyroxene, with minor olivine and iron-titanium oxides. Jasper: Silicon dioxide with varying amounts of iron oxides (hematite, goethite) and other impurities responsible for color.
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