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Jet is a
semi-precious stone which, when polished, takes on an intense waxy lustre of
the deepest
opaque black - hence the use of the term 'jet-black'` in literature since
the eleventh
century. The rich black colour never fades, and the shine which can
be achieved is such
that polished jet was even used as mirrors in medieval times.
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Jet comprises an unusually
pure and hard form of fossilised wood - more specifically
an ancient and relatively
abundant species of monkey puzzle tree - which occurs as thin
lens-shaped seams (fig.1)
within a series of shale rocks (known as the upper lias) which were laid down in the early
Jurassic era some 175-185 million years ago. After these ancient trees had died and
fallen, they would in times of heavy rain and flood often be swept into swollen rivers. On
their way downstream they would be tumbled and battered, with many of the trees being
broken up and/or stripped of their branches. (fig.3)
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fig.3: (A
excellent example of jet showing evidence of its original structure.)
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Upon their eventual arrival
at the sea, the by now water-logged timbers would rapidly
sink into the black mud of the
sea floor, where they would over subsequent millennia
be overlaid with sediments comprised
of sand, mud, and organic remains. It was the accumulated weight of both this sediment and
the water overlying it which exerted the great pressure which, over millions of years and
in chemically-complex conditions,
resulted in the flattening and compressing of the wood
into jet. It is at this point that
a distinction needs to be drawn between 'hard' jet and
'soft' jet; the former being
formed in saline and anaerobic conditions, and the latter
being formed in more
freshwater and aerobic conditions. Only 'hard' jet proper is suitable
for working into jewellery and ornaments, and such finest quality jet occurs only in one
specific and particularly tough and compacted layer of the upper lias shales known as
jet-rock.
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fig.4
(Example's showing ammonites imprinted on the underside of the jet specimen.)
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The ancient lias sea offered
precisely the right conditions for the formation of jet, the
area of this sea' today
corresponds with the area of land encompassed by the North
York Moors National Park. Jet
not only occurs beneath much of the Park landscape
itself, but - more importantly and
accessibly - is also exposed in the cliffs lying to the
north-west and south-east of
the historic fishing town of Whitby in North Yorkshire.
The jet can be found ex situ as small fractured and water-worn pieces, (fig.2),
on beaches and trapped amongst
foreshore rocks, or less frequently in situ as thin
seams within the cliffs themselves.
Also preserved in the Upper Lias shales are the fossilised remains of extinct
sea-creatures such as ammonites and belemnites, which,
if in close enough proximity, can
leave attractive impressions on the
underside of the jet itself. (fig.4)
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