“This composite photomicrograph shows two views of a thin section of crazy lace agate from Sierra Santa Lucia, a classic locality for crazy lace agate. The left view was taken using cross-polarized light, and the right view is in plane-polarized light. This comparison makes it clear why cross-polarized light is so useful for studying chalcedony. The main point of interest is a spherulite that is more complete and more attractive than most, 0.62 mm in diameter (FOV is 2.0 mm on each side of the composite photo). The central portion of the spherulite consists of radiating fibrous microcrystals of quartz. In fact, any such radiating mass of translucent fibrous crystals will produce a pseudo-interference cross such as this, caused by the arrangement of the polaroid plates in the microscope, or "nicols," at 90 degrees to each other. The daughter photo shows a second spherulite only about a centimeter away from this one (i.e, in a different part of this slide--not just the right-hand view here), but that is a long distance in terms of the history of this rock, and this other spherulite has a notably different structure than this one. Crystal spherulites are like snowflakes–their overall structure may be similar, but details differ so that no two are identical, just like all crystals and crystal groupings of any mineral are different. Collection, thin section, and photomicrographs by Norman King.”

“Thin section in cross-polarized light showing another spherulite of chalcedony in crazy lace agate (they're all different!). Spherulite is 0.75 mm in diameter. Collection, thin section, and photomicrograph by Norman King.”