Posted by: frank Post Reply
The " porcelain " jaspers of the American west such as Biggs and Bruneau are other examples of sedimentary jaspers , chert. They are very dense and are much harder of an absolute ( not mohs n) scale than crystalline quartz. The oceanic sedimentary jaspers form from the vast amount of silica falling from dead micro organisms in the ocean water column.They get metamorphosed when the oceanic plate is subducted and get back to the surface when parts of the plate they are in are obducted.The western US jaspers are freshwater deposits of silica leached from basalt then concentrated in voids in the basalt. Oceanic cherts are extremely common in the geology of California , Washington and BC.They take many forms due to various contaminants and how much they were disturbed while forming. Green , blue , black , red and silver cherts are common in the mountains near me in Washington and are often found in blocks hundreds of yards across.Sedimentary rocks are not all porous in fact most " sedimentary "rocks have a sedimentary stage then are cooked either in subduction zones or by regional metamorphism. Hardness has nothing to do with porosity as very hard Brazilian agate is famous for it's porosity , porosity which allowed it to be the basis of the dyed bead trade of Idar.
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