sedimentary
Chert (Yellow/Tan variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, yellow, or mustard; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, yellow, or mustard
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, yellow, or mustard; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) in marine environments. Often found as nodules in limestone or chalk beds.
Uses & applications
Used historically for stone tools due to its sharp edges; currently used in decorative landscaping, construction aggregate, and occasionally tumbled for lapidary use.
Geological facts
Chert is nearly identical to flint, but 'flint' is usually reserved for dark grey or black varieties found in chalk. It is chemically very stable and resistant to weathering.
Field identification & locations
Identify by its 'waxy' texture and conchoidal (shell-like) fracture patterns. It will easily scratch glass and does not react with acid. Commonly found in riverbeds and coastal areas where sedimentary layers have eroded.
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