PRECIOUS STONES. 277
green
forms include a bright green from Brazil called Brazilian Emerald, and
a softer yellow-green type found in Ceylon and known as Ceylonese
Chrysolite or Ceylonese Peridot. Green Tourmaline has a specific
gravity of 3-107. The colourless variety is known as
Achroite, and has a specific gravity of 3'022. Dravite is a brown
Tourmaline from Carinthia.
In
chemical composition Tourmaline is very varied, and in fact it may be
regarded rather as an isomorphous series. It is essentially a silicate
of boron and aluminium, but water and fluorine, as well as the alkali
metals, and the elements titanium, iron, manganese, magnesium, and
calcium may be present. No exact molecular formula has ever been
arrived at; as with many other minerals, an increase in the percentage
of iron present is accompanied by a greater depth of colour.
The
localities where Tourmaline occurs are so numerous that only the places
where material suitable for gem use, or where exceptionally coloured
specimens are found, can be given.
In
the Ural Mountains, in the neighbourhood of Mur-sinka, Bubellite is
found in granite; the blue Indicolite also occurs here, and Bubellite
in the Nerchinsk district of Trans-Baikalia.
In
Brazil in the Novas Minas district and in the Bibeirao da Tolha green
Tourmaline is found, some of it being the bright green known as
Brazilian Emerald; the blue variety " Brazilian Sapphire," and a little
Rubellite also are found here.
The gem gravels of Ceylon yield the yellow and yellow-green stones known as Ceylonese Chrysolite (or Peridot)'
