|
274 PRECIOUS STONES.
decomposition of more or less basic eruptive rocks. It is an acid calcium aluminium orthosilicate,
is
found in many parts of Scotland, as Dumbartonshire and Renfrewshire ;
also with the last mentioned species, Axinite, at St. Christophe in
Dauphiné, at several places in the Alps, at Ala in Piedmont, and in
Cape Colony (" Cape Chrysolite"). In the United States it is found in
the Lake Superior copper region with Native Copper, at Farmington in
Connecticut, and at Bergen Hill in New Jersey. Prehnite is rather apt
to lose its colour on exposure.
426. Tourmaline.
Tourmaline
is almost as remarkable as Corundum for the number of colour varieties
which are used as gems, and as with Corundum, too, these gem varieties
are rarely associated with the name of the mineralogical species to
which they belong. Tourmaline most commonly occurs black, but such
specimens are not used as gems. The various shades of red, blue and
green, in which the mineral is found, are all to be seen in cut
specimens ; more rarely the colourless variety is cut. Only the
transparent forms are used, and these have a vitreous lustre. The
mineral is doubly refracting, but the indices are not high, though
there is a relatively considerable difference between the ordinary and
extraordinary rays, the values of the indices for yellow light being
(in a colourless specimen) 1"637 and 1-619. The dichroism
is, however, very marked, more so than in any other precious stone
except Iolite. The images in the dichroscope are usually of nearly the
same colour as the crystal, but they differ greatly in depth, and
sometimes to some extent in tint ; crystals of the green and blue
|
||
|
|
||