The Pittsburgh Seam of coal has been called the richest mineral deposit ever found, extending from Pittsburgh deep into West Virginia:
http://pubs.usgs.gov/of/1996/of96-280/In the above map, the red areas are the MINED out areas of the seam, the dark gray areas are what remains of the Pittsburgh Seam, the light gray area are "Horizon" areas, areas where Pittsburgh Seam MIGHT exist, but no one knows, or if known of a lower quality.
Mined out parts of the Pittsburgh Seam:
http://en.wikipedia.org/wiki/Pittsburgh_coal_seamThe following site is a detail version of the above map, it does take a lot of bandwidth to download, but goes into details as to the Pittsburgh Seam: Warning this will take several minutes to download:
http://pubs.usgs.gov/of/1996/of96-280/pittsburgh.pdfWhile there are other Coal that are considered good for Steel, the Pittsburgh Seam is considered among the best. Over half of it is gone (If you exclude the "Horizon" areas).