Most people consider tourmaline to be a single mineral.
But in fact it is a group named for several different,
but closely related minerals. Members of the Tourmaline
Group are favorites among mineral collectors. Their
rich and varied colors can captivate the eye. Even the
black opaque tourmalines can shine nicely and produce
sharp crystal forms. Tourmalines are cut as precious
gems, carved into figurines, cut as cabochons, sliced
into cross-sections and natural specimens are enthusiastically
added to many a rock hound's collection. There are many
unique properties of tourmalines. First, they are piezoelectric
which means that when a crystal is heated or compressed
(or vibrated) a different electrical charge will form
at opposite ends of the crystal (an electrical potential).
Conversely if an electrical potential is applied to
the crystal, it will vibrate. Secondly they are pleochroic
which means that the crystal will look darker in color
when viewed down the long axis of the crystal than when
viewed from the side. This property goes beyond the
idea that the crystal is just thicker in that direction.
Even equally dimensioned crystals will demonstrate this
trait. This property can be used as an advantage by
gem cutters who may wish to enhance a crystal's pale
color or weaken a strongly colored crystal. The four
most common and well known tourmalines are distinguished
by their color and transparencies. Elbaite is the gemstone
tourmaline and comes in many varied and beautiful colors.
It is transparent to translucent and is highly prized
as minerals specimens and as gemstones. Elbaite is easily
the most colorful of all the gemstones.
The iron rich schorl is the most abundant tourmaline
and is black and opaque. It is a common accessory mineral
in igneous and metamorphic rocks and can form nice crystals.
Although too opaque to be used as a gemstone, schorl
is used as an ornamental stone when found as inclusions
in quartz, a stone is called "tourmalinated quartz".
Usually when someone refers to tourmaline they are referring
to either elbaite or schorl. The two other more common
tourmalines; dravite and uvite are much less common
than elbaite or schorl, but they are getting noticed
for their beautiful specimens. Some of dravite's crystals
are nicely formed, translucent brown and they can reach
a rather large size. Uvite is a green translucent to
opaque tourmaline that is growing in popularity and
is being cut as a gemstone. The Tourmaline Group has
a general formula of AX3Y6(BO3)3 Si6O18(O, OH, F)4.
The A can be either calcium or sodium. The X can be
either aluminum, iron, lithium or magnesium. The Y is
usually aluminum, but can also be chromium or iron.
Some potassium can be in the A position, some manganese
can be in the X position and some vanadium can be found
in the Y position, but these elements are usually not
represented in the formulas of the tourmaline members.
These are the members of the Tourmaline Group of minerals:
Buergerite (Sodium Iron Aluminum Boro-silicate Hydroxide
Chromdravite (Sodium Magnesium Chromium Iron Aluminum
Boro-silicate Hydroxide Fluoride)
Dravite (Sodium Magnesium Aluminum Boro-silicate Hydroxide)
Elbaite (Sodium Lithium Aluminum Boro-silicate Hydroxide)
Feruvite (Calcium Iron Magnesium Aluminum Boro-silicate
Foitite (Iron Aluminum Boro-silicate Hydroxide)
Liddicoatite (Sodium Lithium Aluminum Boro-silicate
Oxide Hydroxidem Fluoride)
Olenite (Sodium Aluminum Boro-silicate Oxide Hydroxide)
Povondravite (Sodium Iron Boro-silicate Hydroxide Oxide)
Schorl (Sodium Iron Aluminum Boro-silicate Hydroxide)
Uvite (Calcium Sodium Magnesium Iron Aluminum Boro-silicate