Es el término para designar un corte extremadamente delgado de la superficie de una roca (0.003 mm) para permitir el paso de la luz, y poder hacer un estudio en un microscopio petrográfico. El término en Inglés es thin section.

Se utiliza para clasificar a las rocas conforme a su mineralogía (cada especie mineral tiene características especiales y diagnósticas bajo la luz), y tembién se pueden determinar las caracteríticas de esquistosidad, foliación, metamorfismo...

In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral or soil sample for use with a polarizing petrographic microscope. A thin sliver of rock is cut from the sample with a diamond saw or laser, mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 0.03 mm (30 micrometres) thick.

When placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock forming minerals can be easily identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene.

Thin sections are prepared in order to investigate the optical properties of the minerals in the rock. This work is a part of petrology and helps to reveal the origin and evolution of the parent rock.