Gneiss

Gneisses are formed at even higher metamorphic grades than schists. Gneisses have minerals large enough to be identified with the naked eye that have been segregated into roughly parallel bands or layers. These bands may be straight or tightly folded. Many gneisses are made of the same minerals as coarse-grained intrusive rocks such as granite or diorite.

This barn wall shows three large gneisses (black and white/pink striped rocks) plus several coarse-grained intrusive rocks. The one at the lower left is a granite: it has a speckled appearance because its minerals have no particular orientation. The same is true of the gabbro at the top left.
A dike (pink) has cut across this black-and-white banded gneiss. The layers at the top closely resemble the cross-bedding in a sedimentary rock, which is curious.
This is known as an augen gneiss, after the large blobs of stretched feldspar crystals known as augen (German for 'eye'). Note that there is a clear gneissic banding also in this sample.
Here is a small lab sample. It has two pink augen near the bottom, which are probably K-feldspar, plus alternating bands of black (amphibole) and white (probably plagioclase). Click here for a super-close up so you can see how clearly these bands are made of different minerals, as opposed to the bands in quartzites and marbles, which are largely impurities.
Here is a second lab sample, with alternating pink and black layers (K-spar and amphibole) and white and black layers (plagioclase and amphibole). Click here for a super-close up to see the mineral textures.
A good block of gneiss makes the ideal tombstone or kitchen countertop.

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