_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.