QUARTZ is the most common of all minerals, making up about 12 percent of
the earth’s crust. There are two main types of quartz—crystalline quartz
and dense, crypto-crystalline (submicroscopic) quartz. Many dense
varieties occur in Illinois, probably the most common are chert or
flint.
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Crystals of quartz are typically six-sided, elongated, have sharply
pointed pyramid-like ends, and are apt to grow together forming twins.
Good crystals are rare in Illinois, and the crystal structure is not
apparent in the commonly occurring grains and masses.
Quartz is brittle and hard. It may be colorless or tinted, transparent
or translucent, but more commonly it is white and nearly opaque.
Transparent quartz looks much like ordinary glass, but it scratches
glass easily. It has a glassy to brilliant luster and breaks irregularly
or with a good conchoidal fracture.
Some varieties of quartz that are used for semiprecious gems are
chalcedony, agate, onyx, and jasper. Chalcedony is waxy, smooth,
generally translucent, white to gray, blue, brown, or black. Agate is a
form of chalcedony that has a mottled or variegated banded appearance
and may be yellow, green, red, brown, blue, gray, or black. Onyx is
agate with parallel bands that as a rule are brown and white or black
and white. Jasper, an impure opaque quartz, generally is red.
Quartz occurs as rock crystal (colorless, transparent), milky quartz
(white, nearly opaque), and smoky quartz (smoky yellow to gray or brown)
in geodes from the Warsaw and Keokuk Limestones of the
Nauvoo-Hamilton-Warsaw area and as vein and cavity fillings associated
locally with fluorite, sphalerite, and galena in extreme southern
Illinois. It also occurs as vug (cavity) fillings in limestones and
sandstones.