sedimentary
Chert (Holey Stone)
Microcrystalline Quartz (SiO2)
Hardness: 6.5–7 (Mohs); Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Tan, brown, or buff
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 (Mohs); Color: Tan, brown, or buff; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica within sedimentary rock layers, often replacing organic material or carbonate minerals. Many holey stones (hag stones) are formed through the abrasive action of water or boring organisms over thousands of years.
Uses & applications
Used historically for making tools or arrowheads due to its conchoidal fracture. Often kept as lucky charms (hag stones) or used in landscaping and aggregate.
Geological facts
Rocks with naturally occurring holes are often called 'Hag Stones' and have a rich history in European folklore for warding off spirits or protecting livestock.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy texture, and the presence of natural erosion holes. Commonly found on beaches or in stream beds.
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