The photograph shows a specimen of “jet” – although shown in it’s rough state below, jet can be polished like a gemstone, after which it takes on a deep black shine – this is where we get the phrase “Jet black”.
Polished jet jewellery became very popular during the later part of the reign of Queen Victoria, when she wore Whitby jet as part of her mourning dress. There are still many shops there today devoted to the production of jewellery using locally found material.
What is it?
The secret of jet was finally unravelled completely in 1933 by J E Hemingway, a geologist at Leeds University. His research found that driftwood floating in an ancient sea eventually became waterlogged and sank to the sea bed. Here, if lucky, it would be enveloped by the soupy mud there, and in the absence of any dissolved oxygen to cause decay, slowly became fossilised.
Around 182 million years later, the jet we find is heavily compressed wood. In fact, if it is cut into thin slices and examined with a microscope, we can see that the trees that formed it are in fact very similar to the “Monkey Puzzle” tree (Araucaria araucana) which you might find in your local garden centre!
Where does it come from?
Whitby jet can sometimes be found washed up on the beaches of the Yorkshire coast, where it can be identified by it’s colour and the fact that it is much lighter than all the other rocks.
A huge increase in demand in Victorian times meant that demand began to outstrip supply. Geology came to the rescue by identifying the “Jet Rock” in the sea cliffs. The non-geologist miners could easily locate it by finding the “Top Jet Dogger” – a distinctive seam of lighter coloured limestone in the cliffs. Here they would dig small make-shift mines to extract the jet; needless to say this was not a very safe occupation, and many miners lost their lives in search of this elusive material.