sedimentary
Chert (Hagstone/Adder Stone)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5 - 7 (Mohs scale); Color: Variable (tan, pink, gray, brown); Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
- Hardness
- 6
- Color
- Variable (tan, pink, gray, brown)
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5 - 7 (Mohs scale); Color: Variable (tan, pink, gray, brown); Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms or the chemical precipitation of silica in groundwater. The 'hole' is typically the result of differential erosion where a softer inclusion (like a fossil or mineral nodule) has weathered away over thousands of years.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to its sharp edges. Today used in construction aggregate, as decorative garden stones, or as 'charms' in folk traditions.
Geological facts
Rocks with naturally occurring holes are known as 'Hagstones' or 'Adder Stones' in folklore. They were historically believed to protect against the 'evil eye' or to allow the wearer to see into the invisible world of fairies.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), smooth texture, and waxy luster. Commonly found in riverbeds, glacial till, and coastal beaches where softer surrounding rock has eroded away. Collectors look for unique wear patterns and centrally located holes.
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