18. Zircon, Garnet, Staurolites, Tourmaline, and Topaz of
the Brownish-Red Color
There
should be no difficulty in distinguishing these. The lustre of the
zircon is much greater. The topaz shows less brown, the tourmaline
more. Staurolites are seldom met with, and they are very dark. Some of
the almandine and essonite garnets are similar to the zircon, but the
brown tint is not so decided, nor are they as transparent or lustrous,
and they do not show the inclination to yellow, noticeable in the
topaz. The brown or brownish-red tourmaline shows more or less of a
double color, inclining to amber. The di-chroiscope will decide in a
case of uncertainty. Zircon, staurolites, topaz, and tourmaline are
doubly refractive, the two former weak, the two latter strong. Topaz,
yellow and red. Tourmaline, light and dark brown. The garnets are
single refraction.
19. Epidote, Vesuvianite, Axinite, Andalusite, and Cairn-
gorm (in Gray to Brown Shades)
Except
the cairngorm, these seldom come into the market. The following table
will give a clue to the differences existing between them. They are all
doubly refractive, but the colors of the vesuvianite and cairngorm are
indistinct.
Hardness. Specific Gravity. Dichroism.
Epidote.....6.25 3.47 to 3.50 Green, yellow, brown.
Vesuvianite. .6.5 3.4 Indistinct green, yellow.
Axinite......6.75 3.29 to 3.30 Violet, brown, green.
Andalusite.. .7.5 3.17103.19 Yellow, red.
Cairngorm.. .7 2.65 Weak.
20. True Hyacinth (Zircon), Commercial Hyacinth (Essonite), Spinel, and Topaz in Reddish Yellow
The
first of these may be known by its adamantine lustre and specific
gravity (4.6 to 4.7) ; the topaz by its strong dichroism (red and
yellow), and the other two by their single
