BRAZIL.
A
recent letter to the writer from Mr. H. Kilburn Scott, a mining
engineer residing at Rio de Janeiro, gives some interesting notes on
topaz mining at the Ouro Preto locality, in the State of Minas Geraes.
The principal mine—the Boa Vista—has been worked for some years past by
a firm employing about twenty men. The method has been to excavate to
some depth the overburden formed by slides and the caving of the
decomposed inclosing rock and then to run small inclined shafts to
reach the topaz-bearing deposit. There has thus been formed a great
craterlike excavation, in which the clay carrying the topazes has been
followed down some meters below the drainage level. Hence it is
possible to work the mine only during the dry season, and with the
present method the limit of working has probably been reached. To
operate this mine successfully improved methods will be necessary.
CEYLON.
Among
the precious stones gathered from the widely distributed gem gravel of
Ceylon, topaz is fairly abundant, but the bright yellow variety is
absent. What is spoken of as topaz among Ceylon gems is the rarer and
harder oriental topaz, or yellow sapphire. The name of " king topaz "
is also applied to pink or flesh-colored sapphire. The true topazes of
the gravel are either colorless or light green, occasionally also
pinkish-yellow or yellow-brown. The first variety is cut and sold under
the name of water-sapphire, which belongs properly to iolite
(cordierite) ; the name is entirely misapplied, as the true
water-sapphire is blue. The green topazes are sold, with true beryls,
as aquamarines. The pinkish-yellow stones closely resemble those from
Brazil, but with the curious difference that while the latter turn to a
clear pink on being heated (the so-called burnt topaz), the Ceylon
stones are absolutely decolorized by heat. The source of the gems is
not known, as they are obtained only from the gravel, but it must
evidently be in the granite intrusives of the Balangoda group.*
ZIRCON.
CEYLON.'
A
large variety of zircons are found in the gem gravels of the island of
Ceylon, with many other precious stones which are a good deal
confounded among native dealers and classified largely by color. The
Cingalese name toramalli is applied indifferently to both
tourmaline proper and zircon, and special terms are prefixed according
to the color. The green variety is mostly zircon, with some tourmaline
and chrysoberyl. The pale browrn also includes some
tourmaline. Other varieties of zircon found are rich yellow and fiery
red. The readiness with which zircon alters in color by heat is
illustrated in the fact that many greenish stones become a fine yellow
by heating, and that the pale brawn ones are often completely
decolorized in the same manner to form the so-called Matara diamonds.
NEW SOUTH WALES.
A
paper was read several years ago by Mr. D. A. Porter before the Royal
Society of New South Wales on the occurrence of zircon in the Sew
England
a Mineralog. Surv., Ceylon; 1004.
b Ceylon Administration Repts., 1904; Mineralog. Survey.
