Yorkshire Coast Fossils
 

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Seam jet   Beach washed jet


 

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.
 


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)
 
jet example showing its original form  

fig.3: (A excellent example of jet showing evidence of its original structure.)
 

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.
 

 
Ammonites imprinted on underside of jet    Ammonites imprinted on underside of jet

fig.4 (Example's showing ammonites imprinted on the underside of the jet specimen.)
 

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)