sedimentary
Blue Grey Agate Geode
Silicon Dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 6.5-7 (Mohs); Color: Pale blue to smoky grey interior with a tan/brown exterior matrix; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Conchoidal fracture.
- Hardness
- 6
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Pale blue to smoky grey interior with a tan/brown exterior matrix; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None; Conchoidal fracture.
Formation & geological history
Formed by the deposition of silica from groundwater into cavities (vesicles) within volcanic or sedimentary rocks across millions of years. This specimen shows a solid chalcedony fill with a weathered outer rind.
Uses & applications
Commonly used in lapidary arts for bookends, coasters, jewelry cabochons, and as decorative display specimens for collectors.
Geological facts
Agates often form in concentric bands; this specific specimen is more characteristic of a solid chalcedony nodule. The blue-grey color is often a result of the Tyndall effect, where minute particles scatter light.
Field identification & locations
Identify in the field by looking for rounded, potato-like nodules with a rough, waxy skin. They are commonly found in Brazil, Uruguay, and the western United States. Breaking or cutting them reveals the hidden crystal interiors.
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