GEODES are roughly spherical bodies that may be filled with layers of
minerals, lined with crystals, or both. The outer layer of geodes found
in Illinois as a rule is composed of chalcedony, a form of finely
crystalline silica.
Geodes differ from concretions in that they form inward from the outer
shell, whereas concretions develop outward from a center. Even if geodes
have been completely filled by mineral matter, their inward-projecting
crystals prove that they formed within a cavity.
In a partly filled cavity, crystals generally are well formed because
they grew without being crowded. Some of the best mineral specimens
known in Illinois are found as crystal linings in geodes.
[Illustration: uncaptioned]
Quartz is the most common mineral deposited in geodes, but calcite,
aragonite, dolomite, siderite, pyrite, galena, fluorite, and sphalerite
also are found.
Geodes ranging in size from less than one inch to a foot or more in
diameter can be gathered from streams where they have accumulated as
residual boulders after the rock in which they were enclosed has been
eroded.
Hollow geodes are the most desirable because they have better crystals.
They can be distinguished from solid ones by their comparative lightness
of weight.
Geodes are commonly associated with limestone and dolomite, at some
places with shale. In Illinois they can be found most easily in the
Warsaw Formation in the area of Nauvoo, Hamilton, and Warsaw, but they
also occur in other areas and other formations.