Gems - Felspar
( Originally Published 1919 )
(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 formula: KAlSi3O8, NaAlSi3O8, and CaAl2Si2O8 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 1.5 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-lamellae 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 con-sequence 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 passes 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.