Quartz is the dominant mineral found in Keokuk Geodes and shimmering white to transparent quartz crystals fill the walls of many of these geode cavities but 22 additional minerals have also been identified:
 

Aragonite  Occurs as groups of colorless to white crystals, fluoresce a bright green, very rare.

Barite  Crystals range from colorless to white, yellow, pink and pale blue. Barite also occurs as rosettes, clusters of tabular crystals, and long slender crystals that are clear.

Calcite  Occurs as white, colorless, brown, black and rarely pink or orange. Crystals forms include, tabular, dogtooth, nailhead, barrel type and many others. Often different stages of calcite crystallization occur in the same geode. Dark brown calcites occasionally take on a purple and green iridecsence with the inclusion of siderite. 

Chalcedony  A microcrystalline mineral ranging in color from gray, black, blue, orange, red and the rare green. Usually appearing as mammillary (brain or bubble formation). Chalcedony also makes up the geodes outer shell.

Chalcopyrite  Crystals range from bright gold to bronze with striated faces and curved edges.
Usually requires microscope for identification.

Chlorite  Very rare black to gray coatings on crystals.

Citrine  Variety of quartz crystal or cluster that is yellow or orange in color, found filling the entire geode, in bands or surrounding the outer shell, often cut as a gemstone, citrine is somewhat rare in nature.

Dolomite  Pink, brown or buff saddle shaped crystals sometimes dominate the entire geode cavity.

Goethite  Occurs as black shiny elongated crystals and coatings and microscopic inclusions on other minerals.

Gypsum  Crystals are long and clear or colorless also appears as flakes or grains.

Hematite  Very rare but occurs as small or microscopic red crystals, also coatings on other minerals.

Jarosite  Occurs as microscopic yellow to brown granular masses but mostly found as a granular powder or stains. Often confused for sulpher. 

Kaolinite  Bright white to brown clay mineral which coats other minerals, sometimes fills the entire geode cavity.

Malachite  Occurs as small green crystals or as a crust. It is typically found only in geodes
that have or once had chalcopyrite.

Marcasite  Occurs as masses of capillary crystals with a brassy or bronze color sometimes with a greenish tint. Often mistaken for pyrite but marcasite only appears as capillary crystals

Pyrite  (Fool's Gold)  Occurs as gold, reddish or black cubic crystals. Occasionally capillary pyrite crystals form.

Pyrolusite  Occurs as black stains on quartz. Usually requires microscope for identification but some crystals are large enough to see with the naked eye. Pyrolusite ingestion causes the black chalcedony geodes.

Siderite  Occurs as thin coatings that give the gorgeous iridescence to brown calcite.

Smithsonite  Rare but appears as granular gray, brown, blue, yellow or orange masses. Often appears with sphalerite.

Sphalerite  Occurs as shiny black crystals often only one single large crystal. Sphalerite crystals 2 - 4 inches in length have been found.