The Collaborative International Dictionary of English v.0.48:

Gneiss \Gneiss\ (n[imac]s), n. [G.] (Geol.)
   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 gneiss
   or syenitic gneiss. Similar varieties of related rocks are
   also called gneiss.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gneiss
    n 1: a laminated metamorphic rock similar to granite

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "gneiss":
   aa, abyssal rock, basalt, bedrock, block lava, brash, breccia,
   conglomerate, crag, druid stone, festooned pahoehoe, granite,
   igneous rock, lava, limestone, living rock, magma, mantlerock,
   metamorphic rock, monolith, pahoehoe, pillow lava, porphyry,
   pudding stone, regolith, rock, ropy lava, rubble, rubblestone,
   sandstone, sarsen, schist, scoria, scree, sedimentary rock,
   shelly pahoehoe, stone, talus, tufa, tuff