sedimentary
Chert (Chalcedony/Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, and cream; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.65
- Hardness
- 6
- Color
- Brown, tan, and cream
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, and cream; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 - 2.65
Formation & geological history
Formed by the precipitation of silica from groundwater or within sedimentary rock layers like limestone. Found in the Saharan regions of Sudan where ancient volcanic activity and subsequent silification occurred.
Uses & applications
Historically used for stone tools and arrowheads; currently used as semi-precious gemstones, in decorative lapidary work, and for landscaping.
Geological facts
This region of northern Sudan (Bayuda Desert/Nubian Desert) is rich in Neolithic stone artifacts made from similar materials due to its hardness and ability to hold a sharp edge.
Field identification & locations
Identified by its waxy luster, smoothness, and conchoidal (shell-like) fracture. Commonly found as river pebbles or desert lag in ancient drainage basins.
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