Granodiorite
Images (c) 2000, 2003 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Granodiorite, shown here magnified about 2.5 times, has a typical salt-and-pepper appearance. The black minerals are biotite (black mica) and dark-gray hornblende, and the light-colored minerals are yellowish-white feldspar, mostly plagioclase, and translucent gray quartz. The presence of quartz is what makes it granodiorite rather than diorite, and the predominance of plagioclase is what distinguishes it from granite. Although it isn't true granite, granodiorite is one of the granitoid rocks. Rusty colors reflect weathering of rare grains of pyrite, which releases iron. The random orientation of grains suggests that this is a plutonic, not metamorphic rock. This polished slab is from southeastern New Hampshire. The boulder below shows its typical field appearance.
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