Agate is a banded variety of chert. Although the chemical composition of
agate is SiO₂, the same as chert, a microscopically fibrous part of it
having a waxy luster or varying in color or translucency may give the
appearance to the rock that we associate with the name agate. The
mineral name chalcedony is given to the fibrous, waxy material.
Typical agates are most abundant in Missouri in the glacial and stream
gravels in the northern part of the state, although part of the Potosi
drusy quartz and chalcedony in the southeast is also prized. The large
gravel pit near LaGrange, in the northeast, has furnished many beautiful
specimens, not only of agate, but also of petrified wood and fossils.
Missouri lapidists and collectors of semi-precious stones find plenty of
interesting raw material within their own state.