ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE,
SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE
ENSTATITE
(‘_Green Garnet_’)
The small green stones which accompany the diamond in South Africa have
been cut and put on the market as ‘green garnet.’ They are, however,
in no way connected with garnet, but belong to a mineral species
called enstatite, which is a silicate of magnesium corresponding to
the formula MgSiO_{3}; the green colour is due to a small amount of
ferrous oxide which replaces magnesia. The double refraction is biaxial
in character and positive in sign, the least and greatest of the
refractive indices being 1·665 and 1·674 respectively; the specific
gravity ranges from 3·10 to 3·13, and the hardness is only about 5½
on Mohs’s scale. The dichroism is perceptible, the twin-colours being
yellowish and green, and, as usual, is more pronounced the deeper the
colour of the stone. There is also a good cleavage in two different
directions.
With increasing percentage amount of iron enstatite passes into
hypersthene. The colour becomes a dark brownish green, and an increase
takes place in the physical constants, the least and greatest of the
refractive indices attaining to 1·692 and 1·705 respectively, and
the specific gravity ranging from 3·4 to 3·5. Hypersthene is never
sufficiently transparent for faceting, but when spangled with small
scales of brookite it is sometimes cut _en cabochon_.
The name enstatite is derived from ἐνστάτης, an opponent, referring to
the infusibility of the mineral before the blowpipe, and hypersthene
comes from ὑπερσθένος, very tough.
An altered enstatite, leek-green in colour and with nearly the
composition of serpentine (p. 289), has been cut _en cabochon_. It has
much lower specific gravity, only 2·6, and lower hardness, 3½ to 4 on
Mohs’s scale. It is named bastite from Baste in the Harz Mountains,
where it was first discovered.
DIOPSIDE
This species, which is also known as malacolite and alalite, provides
stones of a leaf-green colour which have occasionally been cut. It
is a silicate of calcium and magnesium corresponding to the formula
MgCa(SiO_{3})_{2}, but usually contains in place of magnesia some
ferrous oxide, to which it owes its colour; with increase in the
percentage amount of iron the colour deepens and the physical constants
change. The double refraction is large in amount, 0·028, biaxial
in character, and positive in sign. The least and greatest of the
refractive indices corresponding to the stones suitable for jewellery
range about 1·671 and 1·699 respectively, but they may be as high as
1·732 and 1·750 in the two cases. The specific gravity varies from 3·20
to 3·38, and the hardness from 5 to 6 on Mohs’s scale. Dichroism is
noticeable in deep-coloured stones, but is not very marked.
The name diopside comes from δίς, double, and ὄψις, appearance, in
allusion to the effect resulting from the double refraction; malacolite
is derived from μαλακός, soft, because the mineral is softer than the
felspar associated with it; and alalite is named after the principal
locality, Ala Valley, Piedmont, Italy.
KYANITE
Kyanite, also known as disthene, is interesting for two reasons. Its
structure is so grained in character that the hardness varies in the
same stone from 5 to 7 on Mohs’s scale; it can therefore be scratched
by a knife in some directions, but not in others (p. 79). It has the
same chemical composition as andalusite, both being silicates of
aluminium corresponding to the formula Al_{2}SiO_{5}, but possesses
very different physical characters, a fact which shows how large
a share the molecular grouping has in determining the aspect of
crystallized substances. It is biaxial with small negative double
refraction, the least and greatest of the refractive indices being
1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in
sky-blue prismatic crystals, whitish at the edges, in schist near St.
Gothard, Switzerland. It is seldom cut.
Kyanite is derived from its colour, κύανος blue, and disthene, from its
variable hardness, δίς, twice, and σθένος, strong.
ANDALUSITE
Andalusite bears no resemblance whatever to kyanite, although, as has
been stated above, the composition of the two species is essentially
the same. It is usually light bottle-green in colour, and more rarely
brown and reddish. Its extreme dichroism is its most remarkable
character, the twin colours being olive-green and red. The reddish
gleams that are reflected from the interior are in sharp contrast with
the general colour of the stone, and impart to it a weird effect (Plate
XXIX, Fig. 15). Cut stones are often confused with tourmalines, and
can, indeed, only be distinguished from the latter with certainty by
noting on the refractometer the smaller amount of double refraction
and the difference in its character. The least and greatest of the
refractive indices are 1·62 and 1·643 respectively, and the double
refraction, 0·011, about half that of tourmaline, is biaxial and
negative; the specific gravity is 3·18, and hardness 7½ on Mohs’s scale.
Good stones are found at Minas Novas, Minas Geraes, Brazil, and in
the gem-gravels of Ceylon. It was first known from the province of
Andalusia, Spain, whence is the origin of its name.
IDOCRASE
(_Vesuvianite_, _Californite_)
Idocrase, also known as vesuvianite, is occasionally found in the
form of transparent, leaf-green, and yellowish-brown stones which,
when cut, may be mistaken for diopside and epidote respectively, but
are distinguishable from both by the extreme smallness of their
double refraction. Californite is a compact variety which has all the
appearances of a jade; its colour is green, or nearly colourless with
green streaks.
In composition idocrase is a silicate of aluminium and calcium, the
precise formula of which is uncertain, but may be—
(Ca,Mn,Mg,Fe)_{2}[(Al,Fe)(OH,F)]Si_{2}O_{7}.
The double refraction, which is uniaxial in character and negative in
sign, may be less than 0·001, and never exceeds 0·006, so that it is
not easily detected with the refractometer, even in sodium light. The
refractive indices vary enormously in value, from 1·702 to 1·726 for
the ordinary, and from 1·706 to 1·732 for the extraordinary ray. The
specific gravity varies from 3·35 to 3·45, and the hardness is about 6½
on Mohs’s scale.
The name idocrase, from εἴδος, form, and κρᾶσις, mixture, was assigned
to the species by Haüy, but his reasons have little meaning at the
present day. The other names are taken from the localities where the
species and the variety were first discovered.
Bright, green crystals come from Russia, and also from Ala Valley,
Piedmont, and Mount Vesuvius, Italy. Californite is found in large
masses in Siskiyon and Fresno Counties, California.
EPIDOTE
(_Pistacite_)
Epidote often possesses a peculiar shade of yellowish green, similar
to that of the pistachio-nut—hence the origin of its alternative
name—which is unique among minerals, though scarcely pleasing enough
to recommend it to general taste. Its ready cleavage renders it liable
to flaws; nevertheless, it is occasionally faceted. The name epidote,
from ἐπίδοσις, increase, was given to it by Haüy, but not on very
precise crystallographical grounds.
In composition this species is a silicate of calcium and aluminium,
with some ferric oxide in place of alumina, corresponding to the
complex formula, Ca_{2}(Al,Fe)_{2}[(Al,Fe)OH](SiO_{4})_{3}. It
occurs in monoclinic, prismatic crystals richly endowed with natural
faces. The colour deepens with increase in the percentage amount of
iron, and the stones become almost opaque. The double refraction is
large in amount, 0·031, biaxial in character, and negative in sign.
The dichroism is conspicuous in transparent stones, the twin-tints
corresponding to the principal optical directions being green, brown,
and yellow. The values of the least and greatest of the refractive
indices given by transparent stones are 1·735 and 1·766 respectively;
the specific gravity varies from 3·25 to 3·50, and the hardness from 6
to 7 on Mohs’s scale.
Transparent crystals have come from Knappenwand, Untersulzbachtal,
Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs,
Norway. Magnificent, but very dark, crystals were discovered about ten
years ago on Prince of Wales Island, Alaska.
SPHENE
(_Titanite_)
The clear, green, yellow, or brownish stones provided by this species
would be welcomed in jewellery because of their brilliant and
almost adamantine lustre, but, unfortunately, they are too soft to
withstand much wear, the hardness being only 5½ on Mohs’s scale. In
composition sphene is a silico-titanate of calcium corresponding to
the formula CaTiSiO_{5}, and in this respect comes near the recently
discovered gem-stone, benitoite. The refractive indices lie outside
the range of the refractometer, the values of the least and the
greatest of the refractive indices varying from 1·888 and 1·917 to
1·914 and 2·053 respectively. It is to this high refraction that it
owes its brilliant lustre. The double refraction, which is biaxial in
character and positive in sign, is so large that the apparent doubling
of the opposite edges of a cut stone when viewed through one of the
faces is obvious to the unaided eye (cf. p. 41). Cut stones have
additional interest on account of the vivid dichroism displayed, the
twin-tints, colourless, yellow, and reddish yellow, corresponding to
the three principal optical directions, being in strong contrast. The
specific gravity ranges from 3·35 to 3·45. The negative test with the
refractometer (cf p. 26), the softness, and the large amount of double
refraction suffice to distinguish this species from gem-stones of
similar appearance.
The name sphene, from σφήν, wedge, alludes to the shape of the natural
crystals. The alternative name is obviously due to the fact that the
species contains titanium.
Good stones have come from the St. Gothard district, Switzerland.
AXINITE
Called axinite from the shape of its crystals—ἀξίνη, axe—this species
supplies small, clear, clove-brown, honey-yellow, and violet stones
which can be cut for those who care for a stone out of the ordinary.
The composition is a boro-silicate of aluminium and calcium, with
varying amounts of iron and manganese, corresponding to the formula
(Ca,Fe)_{3}Al_{2}(B.OH)Si_{4}O_{15}. Axinite is interesting on
account of its strong dichroism, the twin-tints corresponding to the
principal optical directions being violet, brown, and green. The double
refraction is biaxial in character and negative in sign, the least and
greatest of the refractive indices being 1·674 and 1·684; the specific
gravity is 3·28, and hardness about 6½ to 7, or rather under that of
quartz.
The best examples have been found at St. Cristophe, Bourg d’Oisans, in
the Dauphiné, France. Violet axinite is a novelty that has come within
recent years from Rosebery, Montagu County, Tasmania.
PREHNITE
This species, which is named after its discoverer, Colonel Prehn, is
found in nodular, yellow and oil-green stones, of which the latter
have very occasionally been cut. It is a little soft, the hardness
being only 6 on Mohs’s scale. The double refraction is large in amount,
0·033, biaxial in character, and positive in sign, the least and the
greatest of the refractive indices being 1·616 and 1·649 respectively;
the specific gravity varies from 2·81 to 2·95. In composition prehnite
is a silicate of aluminium and calcium corresponding to the formula
H_{2}Ca_{2}Al_{2}(SiO_{4})_{3}.
The best material has been found at St. Cristophe, Bourg d’Oisans,
Dauphiné, France.
APATITE
This interesting mineral is found occasionally in attractive green,
blue, or violet stones, but is unfortunately too soft for extensive use
in jewellery, the hardness being only 5 on Mohs’s scale. In composition
it is a fluo-chloro-phosphate of calcium, corresponding to the formula
Ca_{4}[Ca(F,Cl)](PO_{4})_{3}. When pure, it is devoid of colour,
the tints being due to the presence of small amounts of tinctorial
agents. The double refraction is uniaxial in character and negative in
sign, the ordinary index being 1·642 and the extraordinary 1·646; the
specific gravity varies from 3·17 to 3·23. The dichroism is usually
feeble, but sometimes is strong; for instance, in the stones from the
Burma ruby mines (yellow, blue-green). A cut stone might be mistaken
for tourmaline, but is distinguished by its softness, or, when tested
on the refractometer, by its inferior double refraction. It received
its name from ἀπατάειν, deceive, because it was wrongly assigned to at
least half a dozen different species in early days. Moroxite is a name
sometimes given to blue-green apatite.
Beautiful violet stones are found at Ehrenfriedersdorf, Saxony;
Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin County,
Maine, U.S.A.; and blue stones come from Ceylon.
DIOPTASE
Though of a pretty, emerald-green colour, dioptase has never been
found in large enough crystals for gem purposes, and it is, moreover,
rather soft, the hardness being only 5 on Mohs’s scale, and has an
easy cleavage. In composition it is a hydrous silicate of copper
corresponding to the formula CuH_{2}SiO_{4}. The double refraction,
which is large in amount, is uniaxial in character, and positive in
sign, the ordinary refractive index being 1·667 and the extraordinary
1·723. Its colour and softness distinguish it from peridot or diopside,
which have about the same refractivity. The name was assigned to the
species by Haüy, from διὰ, through, and ὄπτομαι, see, because the
cleavage directions were distinguishable by looking through the stone.
Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at
Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli,
near Comba, in the French Congo.