JET.
This
substance is a variety of coal much used in England for mourning
jewellery : it is much blacker, tougher, and harder than the ordinary
Cannel coal, and has a considerable lustre when polished. Its hardness
is 1.5, and specific gravity 1.3, and it has a conchoidal fracture. Jet
is found in detached pieces in clay on the coast of Yorkshire, near
Whitby, on the Baltic coast (where it is called black amber), in the
forest of Ardennes, and in the Pyrenees. Great quantities of the
manufactured jet are sold in Spain and Turkey. This substance is the gagates known
to Pliny, Theophrastus, and other ancient authors, and took its name
from the river Gagus, in Syria, where in ancient times it was found.
Boetius says of this stone, that it secures men from nocturnal fears,
spectres, and ghosts; and Cardanus relates that the saints wore
bracelets and rosaries of this substance to number their prayers. In
manufacturing this material it requires to be frequently moistened with
water, for if it be allowed to get hot by the friction of the cutting
or polishing-wheel, it flies into pieces. It is polished with tripoli
and oil; the final polish being given by the hand, with dry tripoli
powder.
JADE, OR NEPHRITE.
Although seldom used for purposes of jewellery in this country, throughout the whole of Asia this stone
