Opal consists of silicon dioxide, like quartz, plus an indefinite amount of water. "Indefinite" means different samples of opal contain different amounts of water. One may be 20% water and another 10% water. Because opal does not have a crystal structure, it is classified as a mineraloid—a mineral-like substance without well-formed crystals.

Precious opal (not found in Kansas) is a highly valued iridescent gemstone. Opal is widespread in the Ogallala Formation in Clark, Ellis, Logan, Ness, and Rawlins counties. This Ogallala opal is colorless to white or gray and is found with a white, cherty, calcareous rock. Some of it is called "moss opal" because it contains the impurity manganese oxide, which forms dark, branching deposits that look like small mosses in the opal. Moss opal (sometimes called moss agate) has been found in Trego, Wallace, and Gove counties. Opalized fossil bones and shells of diatoms are found in the Ogallala Formation, as is a green opal that acts as a cement in hard, erosion-resistant sandstones.

Chemical compound: silicon dioxide
Chemical formula: SiO2•nH2O (Si = silicon, O=oxygen, n = indefinite amount of water, H = hydrogen)
Color: white, yellow, red, brown, green, gray, blue, or transparent and colorless
Specific gravity: 2.15
Luster: slightly glassy to waxy
Hardness: 5–6
Amount of transparency: transparent to opaque


Buchanan, R., 2010, Kansas Geology: An Introduction to Landscapes, Rocks, Minerals, and Fossils (2nd ed.): Lawrence, Kansas, University Press of Kansas, 240 p.

Buchanan, R., and McCauley, J. R., 2010, Roadside Kansas: A Traveler's Guide to Its Geology and Landmarks (2nd ed.): Lawrence, Kansas, University Press of Kansas, 392 p.

Kansas Rocks and Minerals, Kansas Geological Survey Educational Series 2