Graphite ore.

Source: http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/ archive copy at the Wayback Machine ; English Wikipedia, original upload 3 August 2004 by Chris 73

This image is a work of the "Minerals in Your World" project, a cooperative effort between the United States Geological Survey and the Mineral Information Institute. The images were featured in the "Minerals and Materials Photo Gallery" on the website of the U.S House Subcommittee on Energy and Natural Resources. As a work of the U.S. federal government, the image is in the public domain in the United States.