Jet is a type of coal known as, “lignite,” derived from decaying wood, under extreme pressure. Lignite name comes from “lignum,” the Latin word for “wood,” and lignin’s one of the most common organic compounds found in wood. Jet’s composed of the same elements of living matter … carbon, hydrogen, and oxygen … as the living organism from which it procreated. Gemologists considered it a mineraloid, rather than a true mineral, because it originates organically.
Jet’s black or brown, and may contain pyrite inclusions, apparent as brass colored flakes or a metallic luster. The phrase, “jet black,” comes from its dark color, which, curiously, is not black, but, rather, a brown so dark and dense that it appears to the unaided eye as black!
Jet forms when sunken wood, settles into underwater sediment, where little or no oxygen reaches it, and gets compacted as sediments pile on top of it. This sediment hardens to form “jet rock.”
The quality of jet varies among localities, and, according to the Gemstone Guide G8
“…near the coast in the vicinity of Whitby, Yorkshire, England, … the finest jet has always been found and worked. It occurs in lumps of variable size, often retaining the shape of the branches and twigs of the trees from which it evolved … Spanish jet (found in Aragon, Galicia and Austria) is harder and more brittle than the English product. That from Wurtemberg, Germany, is considered inferior. Material from El Paso County, Colorado, takes a fine polish, and Utah jet (Wayne Country [sic]) is generally an inferior quality with many cracks. Deposits also occur at Pictou, Nova Scotia, Canada but the hardness of the material produced is inferior.” G7
Material from Whitby, in England, dates back to the early Jurassic (Toarcian) age, making it about 182 million years old, and the tree from which this fossilized wood formed closely resembles the Monkey Puzzle Tree G9. Acoma Jet comes specifically from the Acoma Pueblo region.
Because jet forms underwater, the water quality also affects its final condition. Jet formed in salt-water sediments finishes hard, while that from freshwater environments is much softer. The earliest documented use of jet, in art, comes from a 12,000 year old Neolithic period damsel fly larva figure, found in Baden-Württemberg, Germany G6. Likewise, Paleolithic sites in Switzerland and France document ancient use of the mineraloid.
Artists prefer the harder material that originated in salt-water. The purest, jet-black rock, without pyrite or sulphur inclusions nor cracks or other defects makes the best carving material, and both artists and clients prefer compact, dense, hard jet figures and pieces with a bright finish.
Jet has no clearly defined form, making it amorphous, unlike directionally crystalized minerals, and it’s uniform in all orientations: a quality that gemologists call, “isotropic.” It has a greasy dull luster on rough surfaces and a vitreous luster on polished surfaces, which, along with being easy to carve or work on a lathe, makes it an excellent material for making fetishes and other figures. A very interesting characteristic of jet is that, unlike other black materials that one might mistake for jet, like dyed chalcedony, black tourmaline (schorl), garnet (melanite), obsidian, and glass, jet is not cold to the touch.
Contributing Author – Jeffery Bacon