shale

1. (geology) A fine-grained sedimentary rock of a thin, laminated, and often friable, structure.Examples: "As on all large green roofs, the soil is not dirt exactly but a gravel-like growing medium of granulated pumice, shales, clays and other minerals."; "Five years in, ARPA–E’s priorities have shifted, too, for the same reason. The cheap natural gas freed from shale by horizontal drilling and hydraulic fracturing (or fracking) has helped kill off bleeding-edge programs like Electrofuels, a bid to use microbes to turn cheap electricity into liquid fuels, […]"countableuncountable