(DICHROITE, CORDIERITE)
Of
the different names by which this mineral is known, cordierite is in
honor of the French geologist Cordier — while the two others indicate
important characters of the mineral — first, that it is of a violet
color (Greek, ion, violet, and lithos, stone); and second, that it has two colors (Greek, dichroos, two-colored).
When cut as a gem the stone is usually known as water sapphire, or saphir d'eau. In
color it resembles the sapphire closely, although the shade of blue
which it exhibits is that known as Berlin-blue, instead of the
cornflower-blue of the sapphire. The sapphire, however, exhibits nearly
the same color throughout, while a cut stone of iolite, if blue in one
direction, will be seen on turning to be gray in another. On this
account, and by reason of its inferior hardness, it is not prized as
highly as the sapphire, and it has but a limited use. The hardness of
iolite is somewhat higher than that of quartz, being from 7-7.5. Its
specific gravity is nearly similar to that of the latter mineral, being
2.6 to 2.66. In composition it is a hydrous silicate of alumina,
magnesia, and iron. It is barely fusible before the blowpipe, and is
not attacked by acids. Its luster is vitreous, and its color may be
imitated in glass; but the strong dichroism of the native mineral
cannot be copied. It crystallizes in the orthorhombic system; but
clear, transparent crystals are rare, the strong tendency of the
mineral to alter on exposure causing them to become clouded and
opaque. Pieces available for cutting occur generally as grains in
granite, or gneiss, or as rolled pebbles in the beds of streams. /The
finest of the latter come from Ceylon, and this is the source of most
of the iolite used in jewelry/ It occurs similarly in Brazil,
associated with topaz in stream gravels. Good iolite for cutting has
been obtained from granite in Haddam and Guilford, Connecticut, in this
country. Besides blue, iolite may also present colors of yellow, green,
or brown. Only the blue is cut, however, and the cutting is made so as
to show this color at the surface. It is usually given the table, or
step cut, but sometimes the cabochon, especially if, as is sometimes
the case, a star-like effect, like that of the star sapphire, can be
obtained.
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