Greywacke is a kind of a rock. It has not particular usefulness other than
we love the name and it occurs with greenstone in some of the better
vineyards on the North Coast.

For the rock hounds I found the following discriptions on the internet

A texturally immature sedimentary rock containing larger grains in a fine-grained matrix of clay-and silt-sized particles. The larger grains may range from sand- to gravel-sized particles and are composed of quartz, rock fragments and feldspar. In a greywacke, the matrix materials should constitute more than 15% by volume.

These rocks are marine sediments. They may form from a slurry of sediment deposited in deep ocean environments from fast-moving currents. When this is the case, the rocks may exhibit a variety of sedimentary features

GREYWACKE

PROPERTIES
Distinctive features:
Poorly sorted dark grey to greenish, fine-grained sandstone.

Colour:
Various sahdes of dark grey to dark greenish-grey.

Texture and granularity:
Granular, fine-grained.

Composition:
Quartz, plagioclase and tiny rock fragments set in a matrix of microscopic quartz,
feldspar, clay and other minerals that are too small to determine without a microscope.

Field associations:
Formed at bottom of ocean trenches bordering continents by avalanches of submarine origin.
Occurs in association with black shales of deep sea origin.

Varieties:
Feldspathic greywacke, which is rich in feldspar; and lithic greywacke, which is rich in tiny rock fragments.

OCCURENCE
Worldwide, but especially bordering ancient fold mountain ranges.

USES
None of any importance. - EXCEPT GRAPES!