Jet is a compact, black variety of lignite that has been used for centuries as a gemstone and decorative material. Its end use, value, and trade characteristics are entirely different from bulk energy-grade lignite, so it is classified separately — typically under headings for precious or semi-precious stones — rather than with mineral fuels.

Lignite's high inherent moisture — often 25–65% by weight — dramatically reduces its energy density and makes ocean freight costs prohibitive relative to its calorific value. As a result, lignite is almost exclusively consumed near its extraction point, and US import volumes under this heading are typically negligible compared to bituminous coal or petroleum products.