Less commonly, the calcite or dolomite crystals are found on the inside, either alone or associated with bitumen, barite, galena, fluorite, quartz, limonite, sphalerite, pyrite, selenite, or celestite. Geodes exhibit a wide range of sizes from inches in diameter to several feet. Most are less than a foot in width. Geodes differ from concretions in that they are hollow and the crystals grow inward from an outer shell. Concretions are solid and grow outward from a central core.
Concretions are generally noncrystalline.
The Fort Payne and Warsaw-Salem Formations, rock units of Mississippian age, contain numerous geodes in Kentucky. In many places, creeks that drain these formations are filled with geodes. Although often considered a rock, agates are composed of the mineral quartz, and are listed at this web site under quartz . A meteorite is a fragment of stony or metallic interplanetary rock thought to have formed in the asteroid belts billions of years ago when the earth and solar system were first formed. At least 26 small meteorites have impacted Kentucky during recorded times.