A fissile (easy to split) sedimentary silicate-rich rock with particles mostly in the silt and clay size. The color is usually a tone of gray but can be brown, red or green to black. Shale was used from Neolithic times for tools, ornaments, and other purposes.
- Manual of mineralogy, Klein, Cornelis & Hurlbut, Cornelius s. jr. After James D Dana. 21st edition, 1993. Published by John Wiley and Sons, Inc. New York, USA
- 7000 Years of Jewellery, Various Authors, edited by Hugh Tait, British Museum Press, London, 1986. ISBN 9780714150321