sedimentary
Cryptocrystalline Quartz (Agate/Chert)
Silicon dioxide (SiO2)
Hardness: 6.5-7 Mohs scale; Color: Multi-colored (grey, brown, tan, reddish); Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65.
- Hardness
- 6
- Color
- Multi-colored (grey, brown, tan, reddish)
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs scale; Color: Multi-colored (grey, brown, tan, reddish); Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.60-2.65.
Formation & geological history
Typically forms through the deposition of silica from groundwater in cavities of igneous rocks or as nodules in sedimentary limestone/chalk. This specimen shows significant rounding from water transport in a beach or river environment.
Uses & applications
Polished for jewelry, lapidary art, tumble-polishing for collectors, and historical use for making stone tools (flintknapping).
Geological facts
This specimen appears to be a water-worn beach pebble composed of chalcedony or chert. These stones are incredibly durable and can survive millions of years of transport by glaciers and rivers.
Field identification & locations
Identify in the field by its waxy luster, smooth texture compared to surrounding rocks, and conchoidal (shell-like) fracture patterns. Found globally on beaches and in riverbeds.
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