Early in the eighteenth century some children of Holland, playing on a
warm summer’s day in a court yard with a few bright colored stones,
noticed that these possessed a strange power when warmed by the heat of
the sun. They attracted and held (just as a magnet attracts iron) ashes,
straws and bits of paper. On reporting this strange discovery to their
parents the latter, it is said, could give no explanation of the curious
property, but a relic of their knowledge of it is left in the name of
“aschentreckers” or “ash-drawers” which they gave the stones and by
which they were known for a long time.
Such was the method of introduction to the civilized world of the
mineral now known as Tourmaline, a mineral which in variety of color,
composition and properties is one of the most interesting in Nature.
The lapidaries who had given the Dutch children the stones for
playthings did not recognize them as different from the other gems in
which they were accustomed to deal. So to the present day, although
Tourmaline is considerably used in jewelry, it is rarely ever called by
that name. The green varieties are often known as Brazilian Emerald,
Chrysolite or Peridot, some varieties of blue as Brazilian Sapphire,
others as Indicolite, the colorless as Achroite, and the red as
Rubellite, Siberite, and even as Ruby.
It is only somewhat recently that these different stones have been
recognized as being varieties of a single mineral species which is known
by the name Tourmaline. This name comes from a Cingalese word (turamali)
which was applied to the first Tourmaline gems sent from Ceylon to
Holland.
At one time the name Schorl was chiefly applied to the species. This was
before the means of distinguishing mineral species were as well
understood as they are now, and a large number of minerals and even
rocks were included under the name Schorl. One by one, however, they
were distinguished by separate names until Schorl included only
Tourmaline, and shortly afterward the name Schorl was dropped
altogether.
In its opaque form, colored either black or brown, Tourmaline is a
comparatively common mineral. It accompanies many so-called metamorphic
rocks, i. e., rocks which have been changed by heat and pressure from
their original condition, and is also common in granite and other
eruptive rocks. As a rock forming mineral it often occurs as long,
slender prisms, frequently about the size of a darning needle and
radiating in all directions. The only mineral for which it is likely to
be mistaken in this form is Hornblende. It can be distinguished from
this in the following manner: On fusing the powdered mineral with a
mixture of bisulphate of potash and fluor spar (best done on a little
loop of platinum wire) Tourmaline will color the flame green, while
Hornblende will produce no coloration.
The black opaque crystals often reach a large size, as some are known to
be four feet in length. Both black and brown Tourmaline are usually
opaque, and hence have no value as gems. The Tourmalines available for
gems are transparent and have a great variety of color.
The gem Tourmalines are to be found in only a few localities. They occur
in Maine, Connecticut and California in our own country, and also in
Brazil, Russia and Ceylon. The crystals are usually in the form of long,
slender prisms. They often have the peculiarity of being differently
colored in different portions. Thus a crystal may be green at one end
and red at the other, and in cross section may show a blue center, then
a colorless zone, then one of red and then one of green. Some of the
crystals from Paris, Me., change from white at one termination to
emerald green, then light green, then pink, and finally are colorless at
the other termination. In some crystals again the red passes to blue,
the blue to green and the green to black.
[Illustration: TOURMALINE.]
Left Column:
Green Tourmaline (Brazil.)
Green Tourmaline (Haddam, Conn.)
Cross Section of Green Tourmaline (Cal.)
Center Column:
Red Tourmaline or Rubellite (Island of Elba.)
Brown Tourmaline (Gouverneur, N. Y.)
Red Tourmaline or Rubellite in Lepidolite (Cal.)
Right column:
Black Tourmaline (Finland).
Exactly what produces these differences of color is not known. It is
known that black Tourmaline has an excess of iron, the red and green an
excess of sodium and lithium, and the yellow and brown an excess of
magnesium in their composition. These same differences of composition
characterize similar colors in portions of the same crystal as well as
separate crystals. Hence the evidence is quite conclusive that the color
in some way depends on the composition. Many transparent Tourmalines,
while appearing of a uniform color when viewed in any one direction,
exhibit different colors when viewed in different directions. Thus, one
of the long, slender crystals may appear green when held lengthwise in
front of the eye, but when looked at from the end appears brown. Again,
some crystals appear perfectly transparent when viewed perpendicularly
to the sides of the prism, but when viewed from the end are perfectly
opaque. This may be true even when the thickness is less in the latter
direction. Both these properties are due to the arrangement of the
molecules of Tourmaline, which is such as to make the power of absorbing
light different in different directions.
The form of crystals of Tourmaline is usually that of a three-sided
prism. The sides of the prism are usually marked by narrow parallel
lines called striæ, and the prism may be more or less rounded by the
addition of other planes.
If a doubly terminated crystal be examined carefully, it will be seen
that the planes on the two ends are not alike, either in number or
inclination. On one end there may be three planes, on the other six, or
even twelve. If the planes on one end make a blunt termination, those on
the other may make a sharply pointed one. Such a peculiarity of crystal
form is possessed by but few minerals. Those possessing it are said to
be hemimorphic, i. e., half formed. In such minerals it is evident that
the forces of attraction by which the molecules were arranged differed
in character at one end from those at the other. In other words, a
separation of the molecular forces seems to have taken place, one kind
going to one end and the other force to the opposite end. Now, it is a
curious fact that most crystals which exhibit this peculiarity of form
are also pyroelectric, i. e., become electric on heating. It was this
development of electricity which caused the stones with which the Dutch
children played, to pick up ashes, paper, etc., when the stones were
warmed by the heat of the sun. Anyone can repeat their observation by
gently heating crystals or even fragments of Tourmaline and applying
them to bits of paper. The electrical attraction will often be found to
be very strong, though it varies with different crystals. The fragments
should not be overheated, the electricity being most strongly developed
between 100 and 200 degrees Fahrenheit. A study of the kinds of
electricity developed by the heat shows that positive electricity is
produced at one end and negative at the other. Now, this exactly
corresponds with what we have seen of the form of the crystal, and shows
that the polar arrangement of the molecules producing different shapes
at the two ends, also produces corresponding electrical properties.
Crystal form, heat, electricity, and even light, are therefore seen to
have intimate connection, and it may be that this interesting mineral
will furnish us a means of learning more about these forces.
In composition Tourmaline is a complex silicate chiefly of aluminum and
boron. Iron, magnesium, the alkalies, and water also enter in varying
amounts into it. In fact, so complicated is its chemical nature that
perhaps no other mineral has been so often analyzed or had its analyses
so much discussed.
Ruskin, in his “Ethics of the Dust,” thus describes its composition: “A
little of everything; there’s always flint and clay and magnesia in it;
and the black is iron according to its fancy; and there’s boracic acid,
if you know what that is, and if you don’t, I cannot tell you to-day,
and it doesn’t signify; and there’s potash and soda, and, on the whole,
the chemistry of it is more like a mediæval doctor’s prescription than
the making of a respectable mineral.”
As to its hardness and specific gravity, Tourmaline may be said to be
both harder and heavier than quartz, its hardness being 7-7.5 in the
scale of hardness of which the diamond is 10. Its specific gravity is
2.98-3.20. These qualities fit it admirably for use as a gem. It is,
however, quite brittle and even at times friable. Cracks therefore
frequently cut across good crystals and spoil what would otherwise make
a good gem. It is very common to find tourmalines in the rocks broken
into a number of pieces and the fragments “mended” together with quartz
or calcite. This has been true of the black Tourmaline shown in the
accompanying plate. Scarcely any other mineral exhibits this change so
often as Tourmaline, a result due probably to its brittleness and the
character of the rock in which it occurs.
I have shown how one of the most remarkable properties of Tourmaline was
discovered by children. It is also interesting to know that the locality
of the finest Tourmalines in the world was discovered by two boys named
Elijah L. Hamlin and Ezekiel Holmes. They were interested in the study
of minerals and spent much of their leisure time searching for them. One
day in the fall, having been out many hours hunting for new minerals,
they were about to return home when a gleam of something green at the
roots of a tree caught their eye. Eagerly bringing it to light, they
found it to be a beautiful green Tourmaline. A fall of snow that night
prevented their obtaining more of the crystals, but the following spring
they returned and secured many fine gems. For many years thereafter the
locality furnished gems of purest ray serene which have gone to adorn
the coronets of kings and enriched the mineral cabinets of the world.
It is estimated that fifty thousand dollars’ worth of Tourmalines have
been taken from this one locality. Auburn and Rumford, Me., are two
other neighboring localities where good gems have been found. At Haddam
Neck, Conn., fine transparent Tourmalines occur, generally green in
color, and many of them of gem quality. They occur in a granite rock.
The red Tourmaline (Rubellite) from California, illustrated in the
accompanying plate, is found in San Diego County of that State. The
matrix in which it occurs is a lithia-bearing mica (lepidolite) of a
delicate violet color. In this matrix the Tourmaline usually occurs in
radiating masses. The rose color of the Tourmaline contrasting with the
violet of the lepidolite makes an object which is quite a favorite with
mineral fanciers, although the former is not sufficiently transparent to
be used as a gem. At two other localities in the same State large
transparent Tourmalines of varying colors have been found.
The Brazilian Tourmalines are chiefly green in color. They occur in
connection with blue and white topaz. They are the source of the gem
known as Brazilian Emerald, which has not, however, the value of the
true emerald.
The Tourmalines from the island of Elba are generally red in color,
transparent and well crystallized. They are, however, too small to use
as gems.
The Ceylon Tourmalines occur in the gem gravels of that locality
accompanying rubies, sapphires, etc., while those of Russia, chiefly red
in color, come from the Ural Mountains, being found in company with
amethyst, topaz, beryl and other gems.
Oliver Cummings Farrington.