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150 University of California Publications [Geology
beautiful
gem stone that rivals the sapphire in color and excels it in
brilliancy. The color, however, although fairly characteristic, is not
an essential property, for very commonly parts of a crystal are
colorless, while occasionally perfectly colorless small crystals are
found. The color also varies in intensity in different, crystals or in
parts of the same crystal. When pale it is a rather pure bine. When
more intense it assumes a violet tint. In addition to this variation in
color in different parts of crystals, there is a difference at any one
point, depending on the direction in which the light passes. In other
words, the mineral is strongly dichroic. the ordinary ray being
colorless, the extraordinary, blue. A section cut parallel to the basal
plane is practically colorless, while sections parallel to the
principal axis show the deepest color. To get the finest effect,
therefore, gems should be cut with the table parallel to the principal
axis, and this is in contrast to the sapphire, which shows its color
best when cut perpendicularly thereto. If such a section, cut so as to
give the strongest color effects, be examined with a dichroscope. the
contrast between the images is most striking. The image of the
extraordinary ray being freed from the colorless image of the ordinary
ray. presents a remarkable intensity of color, very much deeper, of
course, than can be seen by looking at the mineral in any direction
with the unaided eye. In the lighter parts this color of the
extraordinary ray is a slightly greenish blue inclining to indigo as
it becomes darker, and is very similar to one of the axial colors shown
by some cordierites; but in the more highly colored or thicker parts it
is an intense purplish blue.
The
color is not affected by heat up to the melting point of the mineral.
Fragments heated to a rather bright red and maintained at that heat,
just short of fusion, for five minutes showed no change whatever on
cooling.
Benitoite
occurs generally in individual simple crystals scattered through the
matrix and varying from a few millimeters to about two centimeters
across. The matrix being translucent white, the blue transparent
crystals stand out prominently and often show crystal faces.
It crystallizes in the hexagonal system, trigonal division. The observed forms are the basal plane, the plus and minus
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