much
in demand, and on to dark inky blues that seem almost black by
artificial light. Most sapphires are better daylight stones than evening
stones. Some of the sapphires from Montana, however, are of a bright
electric blue that is very striking and brilliant by artificial light.

HOW SAPPHIRES SHOULD BE CUT. The direction in which the stone is cut
helps determine the quality of the blue color, as the "ordinary" ray
(sapphire exhibits dichroism) is yellowish and ugly in color, and if
allowed to be conspicuous in the cut stone, its presence, blending with
the blue, may give it an undesirable greenish cast. Sapphires should
usually be cut so that the table of the finished stone is perpendicular
to the principal optical axis of the crystal. Another way of expressing
this fact is that the table should cross the long axis of the usual
hexagonal crystal of sapphire, at right angles. This scheme of cutting
puts the direction of single refraction up and down the finished stone,
and leaves the ugly ordinary rays in poor position to emerge as the
light that falls upon the girdle edges cannot enter and cross the stone
to any extent.

To find out with a finished stone whether or not the lapidary has cut it
properly as regards its optical properties one may use the dichroscope,
and if there is little or no dichroism in evidence when looking through
the table of the stone it is properly cut.

Where a sapphire shows a poor color and the dichroscope shows that the
table was laid improperly, there is some possibility of improving the
color by recutting to the above indicated position. However, one must
use much judgment in such a case, as sapphires, like other corundum
gems, frequently have their color irregularly distributed, and the
skillful lapidary will place the culet of the stone in a bit of good
color, and thus make the whole stone appear to better advantage. It
would not do to alter such an arrangement, as one would get poorer
rather than better color by recutting in such a case.

While some of the blue stones about to be described may resemble
inferior sapphires, none of them approaches the better grades of
sapphire in fineness of blue coloration. The scientific sapphire, of
course, does approach and even equals the natural sapphire so that one
must know how to distinguish between them. This distinction is not one
of color, however, and it will be separately considered a little later.

2. Blue spinels are infrequently see