Bone

1. A hard coallike substance high in noncombustible mineral matter; often found above or below, or in partings between, layers of relatively pure coal. Hess
2. In the anthracite-coal trade, a carbonaceous shale containing approx. 40% to 60% of noncombustible materials. Syn: bone coal; bony coal. Hess
3. A tough, fine-grained, gray, white, or reddish quartz. Hess
4. A layer of hard, impure coal which sometimes grades uniformly into the adjacent softer coal and sometimes is sharply separated from it. Bone is usually a mixture of clay shale particles with the coal, the clay particles being well distributed. Kentucky

Source: Dictionary of Mining, Mineral, and Related Terms