Along parts of the coast of California there is a lot of this type of rock. It’s called Greywacke. Greywacke is a type of sandstone, a sedimentary rock which is made from particles (sediment) that have been glued together under pressure. This coarse sandstone consists of poorly sorted, angular to sub angular, grains of quartz and feldspar. It is physically hard and dark grey in colour. Texturally it is an immature sandstone composed of sand mixed with mud.
Graywacke sandstone is made up mostly of sand-size grains that were rapidly deposited very near the source rock from which they were weathered. This type of sandstone contains fewer grains made of quartz and more made of feldspars, volcanic rock fragments, as well as silt and clay, than most sandstone. It is therefore also known as “dirty sandstone.” The volcanic rock fragments give graywacke its greenish-gray colours. In terms of building walls it’s a pretty creepy material to work with. It's basically not a very good stone at all.