TOURMALINE
(_Rubellite_)
Tourmaline is unsurpassed even by corundum in variety of hue, and
it has during recent years rapidly advanced in public favour,
mainly owing to the prodigal profusion in which nature has formed
it in that favoured State, California, the garden of the west. Its
comparative softness militates against its use in rings, but its
gorgeous coloration renders it admirably fitted for service in any
article of jewellery, such as a brooch or a pendant, in which a large
central stone is required. Like all coloured stones it is generally
brilliant-cut in front and step-cut at the back, but occasionally it is
sufficiently fibrous in structure to display, when cut _en cabochon_,
pronounced chatoyancy.
The composition of this complex species has long been a vexed
question among mineralogists, but considerable light was
recently thrown on the subject by Schaller, who showed that all
varieties of tourmaline may be referred to a formula of the type
12SiO_{2}.3B_{2}O_{3}.(9-_x_)[(Al,Fe)_{2}O_{3}].3_x_[(Fe,Mn,Ca,
Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O. The ratios of boric oxide,
silica, and water are nearly constant in all analyses, but great
variation is possible in the proportions of the other constituents.
Having regard to this complexity, it is not surprising to find that
the range in colour is so great. Colourless stones, to which the name
achroite is sometimes given, were at one time exceedingly rare, but
they are now found in greater number in California. Stones which are
most suited to jewellery purposes are comparatively free from iron, and
apparently owe their wonderful tints to the alkaline earths; lithia,
for instance, is responsible for the beautiful tint of the highly
prized rubellite, and magnesia, no doubt, for the colour of the brown
stones of various tints. Tourmaline rich in iron is black and almost
opaque. It is a striking peculiarity of the species that the crystals
are rarely uniform in colour throughout, the boundaries between the
differently coloured portions being sharp and abrupt, and the tints
remarkably in contrast. Sometimes the sections are separated by planes
at right angles to the length of the crystal, and sometimes they are
zonal, bounded by cylindrical surfaces running parallel to the same
length. In the latter case a section perpendicular to the length shows
zones of at least three contrasting tints. In the Brazilian stones the
core is generally red, bounded by white, with green on the exterior,
while the reverse is the case in the Californian stones, the core being
green or yellow, bounded by white, with red on the exterior. Tourmaline
may, indeed, be found of almost every imaginable tint, except,
perhaps, the emerald green and the royal sapphire-blue. The principal
varieties are rose-red and pink (rubellite) (Plate XXVII, Fig. 1),
green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian
sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2),
honey-yellow (Ceylonese peridot), violet-red (siberite), and brown
(Plate XXVII, Fig. 8). The black, opaque stones are termed schorl.
The name of the species is derived from the Ceylonese word, _turamali_,
and was first employed when a parcel of gem-stones was brought to
Amsterdam from Ceylon in 1703; in Ceylon, however, the term is applied
by native jewellers to the yellow zircon commonly found in the island.
Schorl, the derivation of which is unknown, is the ancient name for the
species, and is still used in that sense by miners, but it has been
restricted by science to the black variety. The ‘Brazilian emerald’
was introduced into Europe in the seventeenth century and was not
favourably received, possibly because the stones were too dark in
colour and were not properly cut; that they should have been confused
with the true emerald is eloquent testimony to the extreme ignorance
of the characters of gem-stones prevalent in those dark ages. Achroite
comes from the Greek, ἄχροος, without colour.
[Illustration: FIG. 77.—Tourmaline Crystal.]
To the crystallographer tourmaline is one of the most interesting of
minerals. If the crystals, which are usually prismatic in form, are
doubly terminated, the development is so obviously different at the
two ends (Fig. 77) as to indicate that directional character in the
molecular arrangement, termed the polarity, which is borne out by other
physical properties. Tourmaline is remarkably dichroic, A brown stone,
except in very thin sections, is practically opaque to the ordinary
ray, and consequently a section cut parallel to the crystallographic
axis, _i.e._ to the length of a crystal prismatically developed,
transmits only the extraordinary ray. Such sections were in use for
yielding plane-polarized light before Nicol devised the calcite prism
known by his name (cf. p. 44). It is evident that tourmaline, unless
very light in tint, must be cut with the table facet parallel to that
axis, because otherwise the stone will appear dark and lifeless. The
values of the extraordinary and ordinary refractive indices range
between 1·614 and 1·638, and 1·633 and 1·669 respectively; the double
refraction, therefore, is fairly large, amounting to 0·025, and,
since the ordinary exceeds the extraordinary ray, its character is
negative. The specific gravity varies from 3·0 to 3·2. The lower values
in both characters correspond to the lighter coloured stones used in
jewellery; the black stones, as might be expected from their relative
richness in iron, are the densest. The hardness is only about the same
as that of quartz, or perhaps a little greater, varying from 7 to 7½.
It will be noticed that the range of refractivity overlaps that of
topaz (_q.v._) but the latter has a much smaller double refraction,
and may thus be distinguished (p. 29). Unmounted stones are still more
easily distinguished, because tourmaline floats in methylene iodide,
while topaz sinks. The pyro-electric phenomenon (cf. p. 82) for which
tourmaline is remarkable, although of little value as a test in the
case of a cut stone, is of great scientific interest, because it is
strong evidence of the peculiar crystalline symmetry pertaining to its
molecular arrangement. Tourmalines range in price from 5s. to 20s. a
carat according to their colour and quality, but exceptional stones may
command a higher rate.
Tourmaline is usually found in the pegmatite dykes of granites, but
it also occurs in schists and in crystalline limestones. Rubellite
is generally associated with the lithia mica, lepidolite; the groups
of delicate pink rubellite bespangling a background of greyish
white lepidolite are among the most beautiful of museum specimens.
Magnificent crystals of pink, blue, and green tourmaline have been
found in the neighbourhood of Ekaterinburg, principally at Mursinka, in
the Urals, Russia, and fine rubellite has come from the Urulga River,
and other spots near Nertschinsk, Transbaikal, Asiatic Russia. Elba
produces pink, yellowish, and green stones, frequently particoloured;
sometimes the crystals are blackened at the top, and are then known
locally as ‘nigger-heads.’ Ceylon supplies small yellow stones—the
original tourmaline—which are confused with the zircon of a similar
colour, and rubellite accompanies the ruby at Ava, Burma. Beautiful
crystals, green and red, often diversely coloured, come from various
parts, such as Minas Novas and Arassuahy, of the State of Minas Geraes,
Brazil. Suitable gem material has been found in numerous parts of
the United States. Paris and Hebron in Maine have produced gorgeous
pink and green crystals, and Auburn in the same state has supplied
deep-blue, green, and lilac stones. Fine crystals, mostly green, but
also pink and particoloured, occur in an albite quarry near the Conn
River at Haddam Neck, Connecticut. All former localities have, however,
been surpassed by the extraordinary abundance of superb green, and
especially pink, crystals at Pala and Mesa Grande in San Diego County,
California. As elsewhere, many-hued stones are common. The latter
locality supplies the more perfectly transparent crystals. Kunz states
that two remarkable rubellite crystals were found there, one being 45
mm. in length and 42 mm. in diameter, and the other 56 mm. in length
and 24 mm. in diameter. Madagascar, which has proved of recent years
to be rich in gem-stones, supplies green, yellow, and red stones, both
uniformly tinted and particoloured, which in beauty, though perhaps not
in size, bear comparison with any found elsewhere.