Gabbro and Diabase are dark-colored, coarse-grained, hard igneous rocks,
which may be found in the granite and porphyry regions of southeastern
Missouri and as separate boulders in the glacial deposits north of the
Missouri River. Both resemble basalt, which has been described in detail
elsewhere, except that basalt is fine-grained, whereas gabbro and
diabase are coarse-grained (separate grains easily distinguished without
a magnifying glass). The layman is ordinarily not concerned with the
technical differences between gabbro and diabase, which appear about the
same. Both contain plagioclase feldspar (see FELDSPAR) and a dark green
mineral of the pyroxene family.
[Illustration: Diabase hand specimen. From near Roselle.]
Gabbro and Diabase are sometimes called “black granite.” Their chief use
is as bulk or rubble-stone, although special varieties may be used for
building purposes.