MR. J. T. 1I0TBLACK ON PRECIOUS STONES. 
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protoxide of iron, with no magnesia. This stone is hardly ever 
sold under its proper name, but the green is sold as emerald, the 
red, brown, and black as ruby, the blue, yellow, and white as 
sapphire, and the greenish blue are the French Saphir du Br&nl. 
This stone, in consequence of its general dulness and want of 
lustre, has to be cut thin and set with a proper foil. Though 
classed among the least valuable of the precious stones in Europe, 
in Brazil it is highly valued; and is there worn by the Bishops, &c., 
in place of the true sapphire. 
The green also is principally from Brazil, and is called Brazilian 
emerald. Red tourmaline or rubellite is valuable when free from 
flaws ; the finest known specimen of it is, or was, in Case 40, 
Room III., British Museum, it came from the King of Ava, and 
has been valued at £ 1,000. 
The tourmaline is principally interesting on account of its very 
peculiar physical properties. 
The crystals are generally differently terminated, which is an 
exception to the general law of crystallization, and, in consequence, 
a crystal may bo so heated as to be positively electric at one end 
and negative at the other. The state of polarity may be reversed 
by great cold — cut in slices, it is used in the polariscope to analyse 
the optical properties of other minerals. Two slices cut parallel 
with their axis, and laid one on the other in the same direction, 
are transparent, but if laid in reverse directions they become opaque. 
If a double refracting crystal is placed between two slices of 
tourmaline, the part covered by the crystal is transparent, while 
the rest is opaque. The transparent varieties are generally trap-cut, 
the opaque are faceted both above and below the girdle. 
The lapidary has to remember that this stone is only transparent 
in one direction, and that unless the table is parallel with the axis 
of crystallization an otherwise transparent stone will appear opaque 
in trying to look through it. The specific gravity of tourmaline is 
about 3.0 to 3.3. 
IOLITE. 
Closely allied to tourmaline is iolite, with a hardness of 7 to 7.5, 
and a composition of about silica 48, alumina 31, magnesia 10, 
protoxide of iron 8, but its specific gravity is only 2.6 about. Iolite 
is found in Ireland, Spain, Greenland, and other countries ; it is 
not much employed in jewellery. The transparent variety found 
