sedimentary
Quartz (Variety: Agate / Chalcedony)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs); Color: Pale yellow, honey-orange, or white; Luster: Waxy to vitreous; Structure: Microcrystalline (trigonal system); Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Pale yellow, honey-orange, or white
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Pale yellow, honey-orange, or white; Luster: Waxy to vitreous; Structure: Microcrystalline (trigonal system); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed by the deposition of silica from groundwater in cavities of igneous rocks or sedimentary formations. The region in Luanda, Angola (approx. 8.8S, 13.4E) features Cretaceous sedimentary basins where siliceous nodules and river-worn chalcedony are common.
Uses & applications
Used primarily for lapidary purposes, jewelry-making (beads/cabochons), and as a collector's specimen. In industry, quartz is used for abrasives and glass-making.
Geological facts
Agate and chalcedony are among the most diverse minerals in terms of appearance; their durability makes them common 'river rocks' that survive long-distance transport. The honey-like color is often due to trace amounts of iron oxides.
Field identification & locations
Identify by its waxy appearance, lack of cleavage, and ability to scratch glass. Widely found in riverbeds and coastal regions of Angola as water-worn pebbles.
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