The Mystery & the History Jet of The Black, Black Stone

Jet is one unusual stone. Used as lapidary material for millennia, it gets no respect in mineralogical circles. Among ten common rock and mineral guidebooks, seven don't even list jet in the index. The others? One has a picture with a one-sentence caption. Another offers only a brief paragraph.

- JIM BRACE-THOMPSON

What's up? Well, for one thing, jet isn't officially considered a mineral. A "mineraloid" is as generous as mineralogists allow. One even refers to it dismissively as simply "a rock." Read on, then join my dismay for those who malign such a storied stone.

A GEM WITH ORGANIC ORIGINS

Basically, jet is coal. But no ordinary coal! Most coal was formed in freshwater swamps. Dense mats of vegetation left the immense beds we mine today. Then there's lignite. Lignite also typically formed in freshwater environments but is less fully compressed. Sometimes called "brown coal," it retains evidence of its original woody structure. Jet is often compared to lignite, but try to employ lignite as lapidary material and it crumbles in your hands.

To appreciate the distinct nature of jet, step back to the Jurassic. The conifer Araucaria dotted the landscape. Although currently restricted as a native species to Chile and Argentina, ancient relatives once ranged broadly. It's informally referred to as Monkey Puzzle Tree because an Englishman remarked "it would even puzzle a monkey to climb that tree."

"Classic" jet comes from individual Araucaria trees that fell on land and floated to sea. Once waterlogged, they sank and became buried in muck that eventually turned to claystone. Compressed under the weight of sediments, the trees lost much of their organic content until primarily carbon was left, with a smidgeon of other elements.

Some forms of jet do come from freshwater deposits. These are referred to as "soft jet" versus "hard jet" from marine settings. Deposited as driftwood, jet usually occurs in small lenses recognizable as compressed logs. Like lignite, it retains a degree of its original woody structure, which is sometimes obvious to the naked eye but more often requires magnification.