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Limestone with Common Opal

Submit an Entry: My Sedimentary Rock

By Rextheterrible

Limestone with Common Opal

Common Opal in Limestone

Where This Rock Is From (place, type of locality, etc.)

This specimen is from Clark County, Kansas at the edge of the High Plains.

What This Rock Is

This rock is from the Tertiary System, Pliocene Series (5.3 mya), and Ogallala Formation. It is a sedimentary rock, limestone, containing common opal. The very white area where the red ribbon is located is all opal. I have found in these rocks layers of common opal up to 15 inches thick. There are only a few places in the Ogallala Formation you can find thick layers of common opal.

You do not find the precious jewelry variety of opal in Kansas. Opal is a cousin to quartz and made up of silicon and oxygen. It, however, contains more water and is softer than quartz, but harder than calcite.

What I Like About This Rock

These rocks are very pretty and this is the only place I have found the thick layers of common opal. Most places you only find the small pieces of opal. I like this rock because of its beauty and such a fascinating find of massive common opal.


  • Identifying characteristics: To distinguish common opal from calcite apply a few drops of 5-10% acid and if it actually fizzes you have calcite instead of opal or another silicate. Opal can be white, gray, green, reddish brown, almost clear and banded. Opal has a conchoidal fracture and luster will be from glassy to dull to earthy. It is softer than quartz, but harder than calcite.

Andrew Alden, About.com Geology, says:

Opal is much more common than people think, but it can be hard to identify, and of course experts have a wide range of names for the materials between unaltered diatomite and gem opal. A milky look and moderate hardness are the prime features of common opal.
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