Cave onyx

Cave Onyx is an old name for flowstone.

Onyx was a common term in a certain area of the United States, namely the TAG area and the Ozarks during the 19th and early 20th century. It was used for certain kinds of calcite speleothems which are banded, similar to true onyx.

The calcite concentrations in caves are banded as they are deposited very slowly, layer by layer. The color depends on the climate outside, which changes continually, and the amount of pigments like iron oxide brought in by the water. So they have typically brown, beige, white and transparent banding. Much rarer are other colors like blue and green, which are typically based on copper oxides.

The true onyx is also banded, and looks very similar. The main difference is; onyx is composed of quartz, cave onyx is composed of calcite.

It is easy to determine the difference: with a knife it is possible to scratch calcite but not quartz, several acids show a reaction with calcite but not with quartz. Typically the test with weak hydrochloric acid is used.

There are numerous caves called Onyx Cave.