A chemical weathering product of granite.
Granite is an agglomeration of crystals of quartz, feldspar, mica, and amphibole formed when a body of magma cools underground. When granite is brought to the surface, it is weakest at the boundaries between these crystals. The infiltration of water, as well as chemical processes resulting from bringing two dissimilar materials into contact with each other, result in the slow dissolution of the granite at these boundaries.
From the dissolved minerals, the mica crystals already present in a mass of granite grow. In addition, crystals of other clay minerals form. These inexorably force the cores of the crystals apart, and the rock will disintegrate into a gravel of quartz and feldspar shards, packed in with mica and clay.
This gravelly substance is "grus", and can be found anywhere there is exposed granite. (Do not confuse grus with shards of granite formed from mechanical processes such as exfoliation).