of
the crystal. In such a case some of the incident light is reflected
regularly either from the internal surfaces of the layers which make up
the crystal, or of minute cavities or inclusions therein. When this
chatoyant lustre is very marked it gives us the " asterias " or
star-stones known as star-rubies when red and star-sapphires when blue
or grey : the star-sapphire is the ceraunia of Pliny.
Large
rough crystals of pale blue sapphire from Ceylon, usually waterworn,
but still retaining their hexagonal form, are employed for rock drills
and for other mechanical appliances and instruments. Some of these
crystals weigh as much as a pound avoirdupois. The smaller specimens
often exhibit one or more zones of blue in planes perpendicular to the
principal axis of the prism, while surface striations in the same sense
are very common. Very rarely true twins occur, but not infrequently two
or more crystals are associated by interpenetration. Cavities,
generally of microscopic size abound in these large cloudy crystals :
in a few instances, a liquid with a bubble of gas or vapour may be seen
in a cavity ot large size. The cavities have angular walls, and
occasionally may be regarded as "negative" crystals. What is termed the
"habit" of corundum-crystals differs much in the case of specimens
occurring in different localities. For example, while the Ceylon
sapphires exhibit a form prismatically developed—a hexagonal
bi-pyramid—the specimens from the Helena district on the upper reaches
of the Missouri in Montana, U.S.A., are flat crystals in which the
basal planes of the rhombohedron are conspicuous.
The
hardness of pure transparent corundums,whatever their colour, is
generally given as 9. In reality, there are differences in hardness
between specimens from different localities and of different hues. As a
rule, the true, rich, red ruby can be slightly scratched by white, blue
and yellow sapphire, yet, on the other hand, if a lapidary be
questioned on this subject of relative hardness he may tell the
inquirer that out of ten corundums from rings which he receives to
restore their lost polish, nine will be sapphires and only one a ruby.
