FMF - Friends of Minerals Forum, discussion and message board

[Lou B.]

Can someone tell me what this is and if it is worth anything. Besides really liking it.

[Pete Richards]

This seems to be a septarian concretion. They form by precipitation of minerals in soft sediments that eventually become rock, typically round. The minerals form around a center, which is what gives the round shape. Sometimes an outer edge hardens while there is still a lot of water trapped inside. As this gradually escapes, the inside shrinks and cracks, and later the cracks fill with minerals, which produce the white star you see in yours.

This is a non-commercial site, and we do not deal in the values of minerals or rocks. You will have to turn elsewhere for that.

[Bob Harman]

Your photos show a septarian nodule aka septarian concretion or close variant. Many types and varieties exist, and from many localities.

Google "septarian nodule photos" and you will see many many pictured examples.

This website does not give valuations, but suffice it to say that your example appears to be of very low quality, without much aesthetic collector appeal . I don't think it would have any collector value other than being a knick-knack. BOB

[Bob Carnein]

FYI, the lighter material filling the "cracks" in the concretion is typically the mineral calcite and is often fluorescent in ultraviolet light. Fluorescent specimens are their own collecting category. In some cases, aragonite may also be present and may fluoresce a different color than the calcite. You can test these minerals by taking a small chip and dropping it into a small container of hot vinegar. Calcite and aragonite both "fizz" in weak acid, such as vinegar.

[James Catmur]

I find ones in Scotland where the nodule is an iron-rich limestone and the cracks are siderite (an iron mineral)

James