These mysterious looking formations are taken from what is known as sagenitic agate, which is characterised by straight needles that often radiate in hemispheres and are commonly found at the boundary between the agate and surrounding bedrock. Quite how these needles, let alone structures as intricate in cross-section as these, are incorporated into an agate is far from clear, but it seems likely that they form first in the cavity and are subsequently surrounded by the gel from which the agate forms. As an architect, I find these intricate examples uncannily reminiscent of extruded metal door and window frames!
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