Where is chert deposited?
Chert is a sedimentary rock rich in silica. Franciscan chert is formed from the tiny silica shells (0.5-1 mm) of marine plankton called Radiolaria. Radiolarian chert forms where two conditions are met. First, a deep, open ocean setting is required where there is little continental mud or carbonate sediment to dilute the “rain” of dead radiolarian shells settling to the seafloor. Second, the upper ocean waters need to be relatively rich in nutrients in order for abundant Radiolaria to thrive.
How do geologists get the Radiolaria out of the chert?
The beautiful and intricate Radiolaria tests can be extracted from the chert by crushing it into small pieces and then putting it in a solution of hydrofluoric acid. This is the same acid used to etch glass. The acid dissolves away the less durable rock matrix, leaving the exquisite three-dimensional shells exposed. The tests are then photographed using a scanning electron microscope.