Opalite (Moss Opal)

color photo of opalite sample

Hardness: 5 to 6

Opal consists of silicon dioxide, like quartz, plus an indefinite amount of water. It never forms as crystals. The mineral may be white, yellow, red, brown, green, gray, blue, or transparent and colorless. Precious opal (not found in Kansas) is iridescent and is highly valued as a gemstone. Opal cannot be scratched with a knife, but is slightly softer than quartz. It is found as a lining or filling in cavities in some rocks, as a deposit formed by hot springs, and as the petrifying material in much fossil wood.

A common Kansas mineral, 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 an impurity, manganese oxide, that 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.

Read more about opal in Rocks and Minerals in the High Plains

small drawing of Kansas map with Gove County highlighted in red

The sample pictured above is from Gove County, Kansas

Sources

Buchanan, Rex C., Tolsted, Laura L., and Swineford, Ada, 1986, Kansas Rocks and Minerals: Kansas Geological Survey, Educational Series 2, 60 p.

Klein, Cornelis, 1993, Manual of Mineralogy (after James D. Dana), 21st Edition: New York, Wiley, 681 p.

Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey; text by Liz Brosius, Kansas Geological Survey.

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