sedimentary
Chert (Yellow Jasper/Agatized Chert)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Opaque to translucent yellow-brown due to iron oxide inclusions. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Opaque to translucent yellow-brown due to iron oxide inclusions. Luster: Waxy to vitreous. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians) or by the chemical replacement of limestone/dolomite by silica-rich fluids. Usually found in sedimentary beds or as nodules.
Uses & applications
Historically used for stone tools (knapping). Currently used for lapidary work, jewelry (beads/cabochons), and as a collector's specimen.
Geological facts
Yellow chert is often referred to as 'Yellow Jasper' when it is opaque. It is extremely durable and has been used by humans for hundreds of thousands of years to create sharpened edges due to its predictable fracture pattern.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its waxy texture, and the shell-like 'conchoidal' curves where it has broken. Commonly found in riverbeds or gravel deposits.
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