Jet is a lustrous, intensely black mineral, easily worked and taking a high polish. Jet jewellery has been found in bronze age burial mounds, probably collected from the shore. Jet jewellery was also popular in Roman times and superb examples of Roman era jet have been found in York. Jet was also believed to have curative and magical properties. The Venerable Bede, writing in the eighth century stated
"There is also much jet of fine quality, a black jewel which can be set on fire and, when burned, drives away snakes and, like amber, when it is warmed by friction it holds fast whatever is applied to it."
The Yorkshire, and particularly the Whitby jet industry became prominent in the
second half of the nineteenth century. It reached a peak when mourning wear became
fashionable following the Death of Queen Victoria's consort Prince Albert. At its
zenith in 1870 the jet industry employed over 1,500 people in Whitby in over 200
jet workshops. Mining was a precarious industry, carried out by 'Poor Men's Ventures'
renting the ground they worked on and working on speculation. The Jet was found in
thin seams and the miners were sometimes lowered down the cliff face on ropes to
reach a ledge. Occasionally the remains of pit props can be seen jutting out from
collapsed former adits.
Currently most jet is collected on an ad-hoc basis from shore
exposures supplying a few
workshops (external) operating for the tourist trade in Whitby.
Hard Jet is found in the top 3m. of the Jet Rock (Mulgrave Shale Member). Soft Jet
is found in the overlying bituminous shales. Within the rock jet is found in
isolated masses. Two forms are found 'Plank Jet' and 'Cored Jet'. Plank Jet is
found in the plane of the bedding as elongate masses plano-convex in section.
The largest recorded piece was 1.9m long by 115-140 mm wide by 28mm deep but most
sections are less than 10 mm by 150 mm.
Cored Jet has a spindle shaped section with an irregular silica core.
Jet was formed when logs of conifer like arancarian wood sank into an anoxic
'jet log sea'. There is evidence that the logs were not simply swept out to sea but
were weathered and sometimes developed shrinkage cracks, in which quartz grains
became trapped. After abrasion and stripping of the cortex the logs became
waterlogged and sank to an euxinic sea floor. The wood was delignified diagenically,
The woody elements collapsed and the cell cavities became closed. All the
structural elements were distorted but the plant structure is sometimes preserved
in chalcedonies cored jet where stages from uncompacted wood to altered compressed
jet at the margins can be found.
Tourist Guide to Whitby (external)
References
Rayner, D, H. and Hemmingway, J.E. (eds) (1974) The Geology and Mineral Resources of Yorkshire Leeds. Yorks. Geol. Soc.
Brumhead, Derek. (1979) Geology explained in the Yorkshire Dales and on the Yorkshire Coast David & Charles
North York Moors information service. (1993) Geology of the North York Moors York, North York Moors National Park Information service.
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