is
vitreous, it ranges from transparent to opaque, and is doubly
refractive to a high degree. Its cleavage is perfect on the basal
plane, breaking with uneven fractures. Its specific gravity is from
2.94 to 3.15.
Tourmaline
is one of the most dichroic stones, and individual specimens vary more
from others in composition and proportion than is the case in almost
any other mineral. In colour, black shading to light brown is the
commonest; but blue, green, red, and pink are usually desired. Some of
the shades are very rich; and richness, rather than brilliancy, is the
quality which appeals to the artistic eye of the connoisseur. Curious
specimens have shown internal shades of red and external of green,
while others differ in colour toward the extremities. Dana
distinguishes varieties as follows: rubellite, shades of red,
frequently transparent (two of the finest known specimens of this
variety are in the British national collection in the Natural History
Museum at South Kensington, England); indicolite, indigo blue—Berlin
blue, the Brazilian sapphire of jewellers; Brazilian emerald;
/chrysolite (or peridot),) green and
