Arthroscopic vsorthoscopic

Minerals in thin section which appear dark to black under the microscope when viewed with uncrossed polarizers are called opaque. In very thin thin sections and also in certain orientations some opaque minerals are transparent to light, particularly at their edges, and show a weak brightening into dark, reddish, brownish or smokey greyish colours. Rock-forming opaque minerals are predominantly comprised of ore minerals, graphite and carbonaceous substances.

Pichler, H., Schmitt-Riegraf, C. (1997). Orthoscopic observations. In: Rock-forming Minerals in Thin Section. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1443-8_2