the
more pronounced the dichroism. A constant characteristic of coloured
corundum gems is that they are as beautiful by artificial light as by
daylight.
There
are at least nine varieties of corundum used as gems and familiar to
nearly all jewellers; the coloured varieties, other than the red ruby
and blue sapphire, are named for the gems of other mineral species that
they resemble in colour, only with the distinguishing prefix of
"Oriental." The arbitrary names and colours are: Ruby ("Oriental
ruby"), red; Sapphire ("Oriental sapphire"), blue; Leuco-sapphire
(White sapphire), colourless; "Oriental aquamarine," light
bluish-green; " Oriental emerald," green; " Oriental chrysolite,"
yellowish-green; "Oriental topaz," yellow; "Oriental hyacinth," aurora
red; " Oriental amethyst," violet.
The
colour-varieties of corundum are found in irregular grains and as
crystals embedded in some old crystalline rock, as granite or gneiss.
The gem-varieties frequently occur as secondary contact minerals, which
contact with a molten igneous rock has developed in limestone. These
embedded crystals are frequently liberated by the weathering and
uncovering of such rocks,
