thin section

noun

laboratory preparation rock, mineral, soil, pottery, bones, metal for petrographic microscope

A thin, optically flat sliver of a material, especially a rock or mineral, that can be used in microscopy.
“The purpose of this book is to illustrate the appearance of many of the common rock-forming minerals in thin section under the microscope.”
“Before long, as Dawson (1992) pointed out, Sorby (1863) also made thin sections of some of the fused charges of basalt produced by Sir James Hall.”
Used other than figuratively or idiomatically: see thin, section.