Chrysoprase
The chrysoprase is a gem variety of chalcedony, but which
contains nickel. It is usually pale green, but can also be dark green. It is
made of crystals so small that you can not see under the microscope. This
feature differentiates the amethyst, citrine, and other forms of quartz, which
are more or less transparent and form hexagonal crystals. That's the color it
is sought not for its forms. Because of the shortage of chrysoprase, and its
pleasant green color, chrysoprase is a very popular type of quartz. The purest
gems are comparable to jade, with which it is sometimes confused. Cut into
cabochons is also sought as amethyst.
Use: invisibility.
Citrine
Citrine is a variety of quartz; the yellow color is due to
the presence of trace amounts of iron oxides in the mineral. It is also called
yellow quartz. It is relatively rare in nature. Most often it is heated
amethyst takes a yellow instead of purple.
Cordierite
Cordie rite is more difficult to recognize from the beryl,
especially on surfaces unaltered, as it can be confused with quartz. Surfaces
altered, cordierite is transformed into micro crystalline aggregates of chlorate
and serest, sometimes called pinite, which look "rusty". Cordie rite
appears in the clay sediments undergoing thermal metamorphism. Cordie rite can
also be found in some igneous rocks.
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