“Detail of chalcedony matrix of the agate in thin section using cross-polarized light. This matrix has no special features in itself, but includes numerous crystals of muscovite, perhaps the variety called illite. These are made clear in this view by second-order blue interference color, but are obscured somewhat by being associated with darker “dirty” areas that are relatively rich in material other than microcrystalline quartz. Most in-depth published discussions about chalcedony I have seen mention that fine phyllosilicates (often referred to as “clays”) are likely mixed with the chalcedony as accessory minerals, but I have not seen any photomicrographs published to illustrate that claim. So, here is the proof! Electron micrography might also reveal presence of fine micas, but can only show form (habit) of minerals. In this field of view (2.1 mm) the muscovite crystals range from 0.005 to 0.06 mm (5 to 60 microns). The largest example is near middle of slide at far right. Collection, thin section, and photomicrograph by Norman King.”