ne'is. In mineralogy, a species of aggregated rock, composed of quartz, feldspar and mica, of a structure more or less distinctly slaty. The layers, whether straight or curved, are frequently thick, but often vary considerably in the same specimen. It passes on one side into granite, from which it differs in its slaty structure, and on the other into mica slate. It is rich in metalic ores.

A crystalline rock, consisting, like granite, of quartz, feldspar, and mica, but having these materials, especially the mica, arranged in planes, so that it breaks rather easily into coarse slabs or flags. Hornblende sometimes takes the place of the mica, and it is then called hornblendic / syenitic gneiss. Similar varieties of related rocks are also called gneiss.

Relating to, or resembling, gneiss; consisting of gneiss.

Resembling gneiss; having some of the characteristics of gneiss; -- applied to rocks of an intermediate character between granite and gneiss, or mica slate and gneiss.

Having the structure of gneiss.