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 of 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.
