The tourmaline or precious schorl is known under many different names,
and no other mineral has such a suite of colors.
The colorless variety is known as achroite; the red, as rubellite
or siberite; the blue, indicolite or Brazilian sapphire; the green,
Brazilian emerald; and the yellowish-green, Ceylon chrysolite or Ceylon
peridot. Besides the above colors and their shadings, the tourmaline
occurs in black and brown.
The crystallization is obtuse rhomboid, and generally forms six-,
nine-, and twelve-sided prisms.
Some of the crystals are very large, specimens over eight inches long
having been mined.
The tourmaline crystals are remarkable for their varied and beautiful
groupings of colors. Some are internally blue or brown, surrounded by
a bright carmine red or dull yellow; others are red internally and
are enveloped by a green exterior; crystals are sometimes pink at the
summit and light green at the base, or crimson tipped with black, or
white at one end shading into green and finally into red at the other
end. The hardness of the tourmaline is 7 to 7.5, specific gravity 3 to
3.1, and lustre vitreous.
The tourmaline becomes decidedly electric by heating or rubbing, and
will readily attract small pieces of paper and other small objects. The
rubellite or red tourmaline is composed of:
Silica 42.13
Alumina 36.43
Boracic acid 5.74
Oxide of manganese 6.32
Lime 1.20
Potash 2.41
Lithia 2.04