Sunstone
is the term by which those kinds of feldspar are known which reflect a
spangled yellow light. The appearance comes from minute crystals of
iron oxide, hematite, or gothite, which are included in the stone, and
which both reflect the light and give it a reddish color. Like
labradorite the sheen is visible only when the stone is held at a
certain angle. The sheen of sunstone is best visible when the stone is
held in the sunlight or strong artificial light. The variety of
feldspar to which the sunstone most in use at the present time belongs
is oligoclase, a soda-lime triclinic feldspar. Like labradorite, it
usually exhibits on the surface of easiest cleavage parallel
striations, due to twinning structure. The best sunstone at the present
time comes from Tvedestrand, in southern Norway, where it occurs in
compact masses, together with white quartz, in veins in gneiss. Some
also comes from Hittero, Norway. In Werchne Udinsk, Siberia, another
occurrence was discovered in 1831. Previous to this Bauer states that
all the sunstone known came from the Island of Sattel in the White Sea,
and was very costly, although of a quality which would not now he
deemed desirable. At the present time, although stones of fine quality
can be obtained, sunstone is little used in jewelry, and its market
value is very low. Statesville, North Carolina, and Delaware 'County,
Pennsylvania, are two localities in the United States where good
sunstone has been obtained.
Both
sunstone and moonstone can be accurately imitated in glass, and the
distinction of the artificial from the real by ocular examination alone
might be somewhat difficult. Glass, however, lacks the cleavage of
feldspar, and is somewhat heavier and softer. The discovery of the
method of making artificial sunstone is said to have been accidental,
and was made at Murano, near Venice, when a quantity of brass filings
by chance fell into a pot of melted glass. The product was for a long
time, and is still, used in the arts under the name of goldstone.
Sunstone is sometimes known as aventurine feldspar, in distinction from
aventurine quartz, which presents a similar appearance, owing' to the
inclusion of scales of mica. The term aventurine is from the Italian aventura, meaning
chance, and refers to the chance discovery above referred to. Other
forms of feldspar than those here described occasionally furnish gems
which are transparent and colorless, and valued for their luster. The
varieties chiefly employed in this manner, are adularia, a variety of
orthoclase which is often transparent, the best specimens being
obtained in Switzerland, and oligoclase, in the transparent form in
which it is found near Bakersville, North Carolina.
