Materials & Techniques
Jet is a dense, black, semi precious type of lignite that has a brilliant polish and is often used ornamentally. Considered to be a minor gemstone but not a true mineral, jet is a mineraloid. With an organic origin, jet derives from the extreme pressure decomposition of wood from millions of years ago. Found in two forms, hard jet is created by carbon compression and salt water, whereas soft jet is formed by carbon compression and fresh water. Though jet is easy to carve, it is difficult to create fine details without breaking. The similar appearance of anthracite (black coal) and ebonite (hardened rubber) make these minerals difficult to distinguish from jet and are often used as a popular choice to for jet imitation.
Getty Vocabulary, AAT (jet (coal): AAT: 300045514)
"Jet (lignite)," Wikipedia, accessed May 26, 2015, http://en.wikipedia.org/wiki/Jet_(lignite).