Jet
Composition: a variety of brown coal or lignite. Structure: woody. Hardness: 3 to 4. Specific gravity: about 1.30 to 1.35. Luster: dull. Color: black, brownish black. Streak: brown to brownish black. Cleavage: none. Fracture: uneven to smooth. Tenacity: tough to slightly brittle. Diaphaneity: opaque. Burns with a sooty yellowish flame.
Jet is a type of fossil wood in which there has been sufficient chemical change to make the wood relatively hard and black without destroying the woody structure. The best specimens of jet polish into lustrous black cabochons.
Jet occurs in Presidio County as compressed and flattened trunks of trees in a thin layer of coal and lignite in Cretaceous strata 100 to 200 feet stratigraphically below the San Carlos beds.
Specimens of “jet” have been found in some of the lignitic Tertiary strata of the Texas Gulf Coastal Plain; however, this material is mostly soft, brownish, and not of gem quality.