15 September 1995
Vol 269, Issue 5230

About The Cover

A section of iris agate (75 micrometers thick) as viewed in cross-polarized light. Blue bands alternate with more refractive pink bands over several length scales. The banding reflects modulations in the density of defects within fibers of chalcedony (a form of quartz) oriented perpendicular to the banding. The field of view measures ˜2.9 millimeters across. See page 1562. [Photo: P. J. Heaney. Specimen from the Mojave Desert, California, U.S. National Museum specimen number 87415]