Jet
is a variety of coal which, being compact, takes a good polish, and
hence can be used in jewelry. Its hardness is between 3 and 4, and
specific gravity 1.35. It is a kind of brown coal or lignite, and
retaining as it does some of the original structure of the wood, is
not brittle and smutty as is most coal. To be of the quality desirable
for cutting it must be black, of a uniform color, and have a somewhat
fatty luster.
The
jet of commerce has for a long time come chiefly from Whitby,
Yorkshire, England. It occurs here as layers in schists of Upper Lias
age. The industry of mining and cutting the jet has at times reached
extensive proportions. In 1855 twelve hundred to fifteen hundred
artisans were employed in the work, and the annual value of the output
was $100,000. While Whitby is still the center of the industry, the
demand for jet has considerably decreased, and the trade has suffered a
serious setback. The jet manufactured in England is not all of local
origin, much of it being obtained from France, Spain, Italy,
Wurtemberg, and the Orient. Near the close of the eighteenth century
considerable cutting of both foreign and domestic jet was carried on
in France, but the industry is now largely abandoned. Good jet occurs
in numerous localities in America, especially in Colorado, and in
Pictou, Nova Scotia, but it cannot be cut profitably to compete with
the English product. In the anthracite coal regions of Pennsylvania
this variety of coal is cut into a great variety of objects, which find
a more or less extensive sale. Jet is employed chiefly for mourning
jewelry. The decline in its use has come partly from a loss of its
popularity and partly from the substitution for it of black onyx or
black glass. These latter can be prepared somewhat more cheaply than
jet, and while sometimes fraudulently substituted for that mineral, are
often preferred when an opportunity for a choice is given. If it is
desired to distinguish jet from either of these, it can be known by
being softer and lighter, and by having a warmer feeling in the hand.
Hard rubber and celluloid are also sometimes substituted for jet, in
which case they can be distinguished by the fact that jet does not give
a shaving under the knife, but crumbles away. The manufactured articles
are usually also given their form by being pressed in molds, and by
close inspection traces of the molds can be seen.
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