Dichroite is sometimes known under the mineralogical names of
cordierite and iolite, and commercially as _saphir d’eau_, or water
sapphire. This stone is remarkable for pleichroism, sometimes showing
three different colors in as many directions, and when properly cut has
often the star formation of the corundum star-stones.
Water sapphire, as the blue specimens are called, is 7 to 7.5 in
hardness, specific gravity 2.56 to 2.67, transparent to translucent,
and frequently full of flaws. It is partially decomposed by acids,
melts with difficulty before the blow-pipe, is vitreous to greasy in
lustre, and is composed of:
Silica 49.
Alumina 32.
Ferrous oxide 7.
Magnesia 9.
Besides the _saphir d’eau_, which is blue, dichroite occurs colorless,
bluish-white, yellowish-white, yellowish-gray to yellowish-brown,
indigo to blackish-blue, and violet. This mineral is found in Ceylon,
Spain, Norway, Sweden, Tuscany, Greenland, and Bavaria. Sapphire is
harder and much heavier than dichroite.