The
carbon of which we speak can be cut into facets on the mill like the
diamond, and, in that case, it reflects light while still remaining
perfectly opaque.
It is found in great abundance, and therefore can be had for from six to seven lire the carat.
Its
dust is preferable to that of the diamond, on account of its low price,
and is found exceedingly successful in engravings on pietra dura and in cutting cameos.
We must not mistake this substance for the adamantine spar of China.
XXI.
CYANITE.
The name of this stone comes from the Greek κυανό«, blue, which
was given it on account of its colour. It has been known in various
ages, and there is a tradition that it was wrought in England, under
the reign of James I., by one Cornellius, a German artist, who called
it seppara, which name it still retains in France.
The tint of the cyanite is Prussian blue, and occasionally it fades from that to grey or green.
It
does not melt under the blow-pipe. It is transparent, and presents
prismatic reflections, especially when cut smooth. Its specific weight
is 3.5. It is not very hard, but scratches glass with its angles.
