opacite
/ˈəʊpəsʌɪt/Geology
noun
Microscopic dark grains and scales occurring in the groundmass of many igneous rocks, which are too small to be readily identified by optical means and are sometimes attributed to iron oxides or carbon.
Origin
Late 19th century; earliest use found in Edward Dana (1849–1935), mineralogist and physicist. From German Opacit from classical Latin opācus + German -it.