He also made many experiments with the tourmaline when cut into thin slices, and reduced to the finest powder, in which state each particle preserved its pyro-electricity; and he showed that scolezite and mesolite, even when deprived of their water of crystallization and reduced to powder, retain their property of becoming electrical by heat.
In blue tourmaline and in iolite - stones sometimes mistaken for sapphire - the dichroism is much more distinct.
He found that the electricity of the tourmaline decreased rapidly from the summits or poles towards the middle of the crystal, where it was imperceptible; and he discovered that if a tourmaline is broken into any number of fragments, each fragment, when excited, has two opposite poles.
Flattened crystals of garnet, films of quartz, and needles of tourmaline are not uncommon.
The granite, which is intruded through the Eocene beds, is associated with a pegmatite containing tourmaline and cassiterite.
In other cases, especially near mineral veins, slates are filled with black needles of tourmaline or are bleached to pale grey and white colours, or are silicified and impregnated with mineral ores.
Other minerals which occur in the rocks of this group are calcite, garnet, biotite, chloritoid, epidote, tourmaline and graphite or dark carbonaceous materials.
In Oxford county tourmaline, spodumene (or kunzite) and beryl occur, the tourmaline crystals being notably large and beautiful.
Other valuable stones, the topaz, chrysolite, aquamarine amethyst and tourmaline are found.