sedimentary
Oolitic Chert
Oolitic Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan to mustard yellow with cream ooids; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan to mustard yellow with cream ooids
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan to mustard yellow with cream ooids; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed in warm, shallow, agitated marine environments where calcium carbonate ooids were deposited and later replaced by silica (silicification) during diagenesis. Most common in Paleozoic limestone formations.
Uses & applications
Used primarily by rockhounds for tumbling and lapidary work (cabbing). Historically used by Indigenous peoples for tool making (knapping) due to its sharp edges when broken.
Geological facts
The small round structures, called ooids, originally formed like tiny pearls by layering minerals around a central grain of sand or shell fragment in moving water.
Field identification & locations
Identify in the field by looking for the distinct 'fish egg' pattern of ooids within a hard, smooth-textured matrix. It will scratch glass and will not fizz with acid, unlike oolitic limestone.
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