registering
small variations in pressure, and is useful in the estimation of depth
of submarines and the tourmaline is more valuable for this property
than as a precious stone.
Composition is approximately (Mg, Fe, Mn, Ca, Na, K, Li, H)
. Sub-species could not be determined
by
composition as in garnet, although chemical analysis may help to divide
into iron-tourmaline, magnesia-tourmaline, lithia-tourmaline, etc., but
classification has been based on colour.
Uses
Gem
tourmaline is usually cut with the table parallel to the vertical axis,
which shows the dichroism and a pleasing variation in colour. It is
appreciated as a good stone for pendants, brooches, etc. The
piezo-electric property of tourmaline has already been referred to.
Grades
Gem tourmaline is appreciated as a good stone for pendants, brooches, etc.
As already mentioned, tourmaline may be divided into iron-, magnesia-, and Uthia-tourmalines.
The usual names based on colour for the gem varieties are as follows:—
1. Achroite—colourless.
2. Shorl—black.
3. Rubellite—red.
4. Indicolite—indigo-blue.
They
are also called ' Brazilian ruby',' Brazilian emerald', and ' Brazilian
sapphirine'; these terms are misleading and should be dropped. The
colours should be mentioned along with the tourmaline. By heat
treatment soma tourmaline could be unproved. Tourmaline, rich in
alkalies, is transparent and has the most arractive colours.
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