These microscope images are part of the micROCKScopica project, which is an exploration of the aesthetic power of polarized light microscopy, created and maintained by Bernardo Cesare of the University of Padua’s Department of Geosciences. He takes photos of a wide variety of minerals and other objects, such as this mineralized dinosaur bone (left) and Brazilian agate (right). Surprisingly, these samples have largely the same chemical makeup: they’re both chalcedony, a kind of microcrystalline quartz (aka silicon dioxide). Because chalcedony crystals are deposited in small fibers by aqueous solutions of SiO2, they’re able to get into the bone and reproduce the bone’s inner pores and structures. In the agate, the fine chalcedony fibers grow radially outward and create its rounded “spherulitic” texture. The black parts of the agate and fossil probably contain some iron hydroxide inclusions and some skeletal phosphates, respectively.
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We updated this story on Jan. 24, 2020, because "left" and "right" were flipped in the text.