of
hardness, besides the difference in colour, distinguish the sapphire
from the ruby; otherwise the sapphire's chemical and physical
characteristics are generally included in the description of corundum
in the foregoing chapter, covering the red corundum and other red
stones termed rubies. While the form of the sapphire crystal
corresponds with that of the ruby, there is a difference in the habit
of crystallisation; the prism and rhombohedron of the ruby, are
replaced in the sapphire by the hexagonal pyramid. The colouration of
sapphires is frequently irregular; different portions of the same stone
show different colours, and sometimes the body of what would be a
colourless sapphire shows blue patches; but as the blue colour vanishes
when the stone is heated, such a stone, undesirable as a gem, can be
rendered valuable by heating it until it becomes a clear white
sapphire. The colours of sapphire range from the white, colourless, or,
so-called, " Leuco-sapphire "; through the yellow, called " Oriental
topaz "; and through various tints to the royal blue of the typical gem
sapphire. Sometimes sapphires show different colours at their
terminations, as greenish-blue at one end and blue at the other, or
red and blue at the ends; examples
