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826
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MINERAL RESOURCES.
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during
the breaking down and washing away of its loose matrix. The latter
takes place readily when the protecting cover of hard sandstone is
removed. In one place near the garnet-bearing formation some ant hills
were found to be built up of over half garnet chips ranging up to 2 or
3 millimeters across. The remainder of the mineral grains of these ant
hills consisted chiefly of cleavage chips of orthoclase feldspar, with
a smaller amount of hornblende gneiss rock fragments, grains of quartz,
epidote, etc.
The
garnets found in this region range in size from small grains to over 3
centimeters in diameter. The larger ones are not perfect, being badly
flawed and cracked. They often have a brownish-red color, and rarely
contain gem material. The best gem garnets are not often over a
centimeter or 12 millimeters in diameter, and the greatest yield of
gems is in garnets of less than 8 millimeters in diameter. Garnets
that will cut perfect gems over 3 carats in weight are scarce, while
those ranging from 1 to 2 carats when cut are fairly plentiful. Stones
of 1 carat and under are abundant. A garnet cut as an ordinary
brilliant measuring 8 millimeters across and 5.5 millimeters thick
will weigh about 2-1/2 carats. The garnets range in color from
the beautiful rich Burgundy wine red characteristic of pyrope to
lighter shades, with some of more or less cinnamon color. Some of the
red garnets are so dark that the gems show little color and appear
nearly black. In others the colors appear brilliant even under
artificial light.
Imperfections
occur in many of the garnets. These imperfections may be cracks or
flaws or inclusions of dark spots or of other minerals. In the
deeper-colored garnets flaws are often difficult to detect until the
stone has been partly or wholly cut. The dark spots appear to be due to
cavities in the garnet in some cases; in others they are caused by
inclusions of other minerals. Among the minerals found associated with
the garnets is emerald-green diopside, and in one specimen of garnet
which had been split a small diopside crystal of pin-head size was
found in the center. In another specimen a tiny garnet was found
attached to a larger diopside crystal. Minute acicular inclusions are
also present in some of the garnets. They are arranged according to
certain directions of symmetry in the crystal and are probably rutile.
These acicular or threadlike inclusions are so fine that they do not
perceptibly affect the color and luster of the gems.
The
garnets are collected by the Indians who search carefully over the
sandy country below the drift. Apparently no methods are used to find
the gems that may be concealed below a few inches of sand, but only
those on the surface are looked for. The shifting of the sand uncovers
garnets at one time and covers them up at others, and therefore renders
the possibilities of new finds attractive. No water can be obtained
except in rainy seasons to wash for the garnets, so that this method of
concentration can not be used. By the use of screens it seems possible
much material could be worked over with good results. The sand could be
thus eliminated, and the garnets are rather easily picked out from
other pebbles. Screens varying in size of mesh from four-fifths to
one-fifth of an inch would be very serviceable in screening and
separating the material for hand picking. In the majority of cases the
garnets are richest near the bottom of dunes or sand beds, and that
portion should therefore be sieved.
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