Coal is a combustible rock, rich in carbon and formed by compaction of fossil plant remains similar to peat. Thin veins in this piece are filled with pyrite , an abundant ore of sulfur . Limonite is a distinctively yellowish brown ore of iron . It takes many forms, including the cellular structure seen in this sample from the historic Iron Hill area near Waukon in Allamakee County. This pyramid crystal of translucent calcite is from Mahaska County.
Calcite is the principal mineral in limestone, chalk, and marble.
It occurs in a variety of colors and bubbles vigorously when a drop of dilute hydrochloric acid is applied. They include the prized Lake Superior agates, known for their fine, alternating bands of rich colors. Barite is an unusually heavy mineral. This sample from Fayette County is composed of rounded masses of radiating crystals. Barite is used primarily as an additive in drilling muds and paints.