sedimentary
Green Banded Chert (Agate/Jasper)
Silicon Dioxide (SiO2) with trace impurities of Chlorite or Glauconite
Hardness: 6.5-7 (Mohs scale); Color: Olive to forest green with dark grey/black bands; Luster: Waxy to vitreous (especially when wet or polished); Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Olive to forest green with dark grey/black bands
- Luster
- Waxy to vitreous (especially when wet or polished)
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Olive to forest green with dark grey/black bands; Luster: Waxy to vitreous (especially when wet or polished); Crystal Structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or as water-worn pebbles in riverbeds and coastlines. Many specimens date from the Paleozoic to Mesozoic eras.
Uses & applications
Used primarily for lapidary work (cabochons, beads), decorative garden stones, and historically for stone tools due to its sharp conchoidal fracture. Sometimes used as a source of industrial silica.
Geological facts
Chert and its variants like jasper were the 'steel' of the prehistoric world; their ability to be knapped into sharp edges made them essential for survival. Green varieties often get their hue from iron-rich silicate minerals that are incorporated during the replacement process.
Field identification & locations
Identify in the field by its hardness (it will easily scratch glass), smooth waxy texture, and the distinctive 'scalloped' or conchoidal chips on the edges. Commonly found in gravel pits, mountain streams, and along beaches where sedimentary layers have eroded. Collectors should look for distinct banding and lack of visible grains.
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