Posted by: frank Post Reply
This stone is a meta-chert, a chert which has been metamorphosed yet again. The black layers are various iron minerals and the yellow areas are a type of manganese garnet, spessartite. The garnet crystals are very small and aggregated in bunches that the geologists call garnet trains. The entire rock is over 90% silica, a hard jasper. In the field the outside of these metacherts are covered with spalling from collisions in the glacial flow and/or stream beds. This makes the rock look less saturated in color than it really is inside.
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