FELSPAR
(_Moonstone, Sunstone, Labradorite, Amazon-Stone_)
Though second to none among minerals in scientific interest, whether
regarded from the point of view of their crystalline characters or
the important part they play in the formation of rocks, the group
included under the general name felspar occupies but a humble place
in jewellery. It consists of three distinct species, orthoclase,
albite, and anorthite, which are silicates of aluminium, and potassium,
sodium, or calcium, corresponding to the formulae KAlSi_{3}O_{8},
NaAlSi_{3}O_{8}, and CaAl_{2}Si_{2}O_{8} respectively, and also of
species intermediate in composition between albite and orthoclase, or
albite and anorthite. While differing in crystalline symmetry, all
are characterized by two directions of cleavage which are nearly at
right angles to one another. The double refraction, which is slight in
amount, is biaxial in character and variable in sign. The values of the
least and greatest of the indices of refraction range between 1·52 and
1·53, and 1·53 and 1·55 respectively, the double refraction at the same
time varying from 0·007 to 0·012. The specific gravity lies between
2·48 and 2·66, and the hardness ranges between the degrees 6 and 7 on
Mohs’s scale.
Moonstone (Plate XXIX, Fig. 4), which is mainly pure orthoclase, alone
is at all common in jewellery. It forms such an admirable contrasting
frame for large coloured stones that it deserves greater popularity;
no doubt the cheapness of the stones militates against their proper
appreciation. The milky, bluish opalescence from which they take their
name is caused by the reflection of light at the thin twin-lamellæ
of which the structure is composed. They are always cut more or less
steeply _en cabochon_. The finest stones were at one time cut from
the felspar that came from the St. Gothard district in Switzerland
and was in consequence known as adularia from the neighbouring Adular
Mountains, somewhat incorrectly, since none occurs at the latter
locality. At the present day practically all the moonstones on the
market come from Ceylon. They run in price from £3 to £20 per oz. (28
grams).
Sunstone is a felspar containing flakes of hematite or goethite which
impart a spangled bronze appearance to the stones. Good material occurs
in parts of Norway. The remarkable sheen of labradorite or blue felspar
has its origin in the interference of light at lamellar surfaces in
the interior; the uniformity of the colour over comparatively large
areas testifies to the regularity of the lamellar arrangement. The
finest specimens were brought from the Isle of St. Paul off the coast
of Labrador, where they were first discovered in 1770; large masses
also occur on the coast itself. Amazon-stone is an opaque green felspar
which occurs in the Ilmen Mountains, Orenburg, Russia, and at Pike’s
Peak, Colorado, United States. It obtains its name from the Amazon
River, where, however, none has ever been found; there may have been
some confusion with a jade or similar stone.
Occasionally clear colourless felspar has been faceted, and then
closely resembles rock-crystal. A careful determination of the
refractive indices and the specific gravity serves to discriminate
between them.
[Illustration: _PLATE XXIX_
1. CAT’S EYE
2. PERIDOT
3. PERIDOT
4. MOONSTONE
5. HESSONITE
6. PYROPE
7. DEMANTOID
8. ALMANDINE
9. SPODUMENE
10. KUNZITE
11. HIDDENITE
12. ZIRCON
13. ZIRCON
14. ZIRCON
15. ANDALUSITE
16. NEPHRITE
17. TURQUOISE
18. JADEITE
GEM-STONES]