small.
Ordinary Chrysoberyl is but feebly dichroic, but Alexandrite shows
marked dichroism even to the unaided eye; when viewed in artificial
light in the direction of the shortest crystallographic axis the colour
is columbine-red, though in daylight the general tint is green ; and in
this direction, too, the dichroscope gives two distinctly coloured
images, one emerald green and the other yellow, while in another
direction a columbine red colour is seen. The Alexandrite is not only
dichroic, but trichroic.
Alone
it is infusible before the blowpipe and its colour remains unchanged
when heated. When rubbed it shows a charge of positive electricity,
which is retained for longer than is usually the case with minerals.
The
specific gravity is 3"68 to 8'75; the fracture is conchoidal to uneven;
it has a distinct cleavage parallel to one and cutting the other two
crystal axes; it is brittle but very hard, ranking in point of hardness
between the Topaz and Sapphire, or equal to 8-1/2. The streak is
colourless.
In
crystalline form it is rhombic and when in single crystals its habit is
tabular, the faces normal to the shortest axis being largely developed
and showing striatums parallel to the vertical axis. More often the
crystals are twinned, either in three-lings or six-lings, so as to give
a pseudo-hexagonal appearance to the group.
The
variety Cymophane shows an interesting minute structure ; this consists
of a multitude of very small pores or cavities in the crystal arranged
in a definite way, giving a certain degree of cloudiness to these
specimens and also causing the opalescent effect seen on moving the
stone. When the band of light is very sharply defined the stone showing
it is called a Cat's Eye.
