FMF - Friends of Minerals Forum, discussion and message board

[dgc]

I've got several round & oblong rocks found in a field when I was young. They are similar to sandstone, kinda soft. Some are totally hollow; some have what sems like flint rock inside. What are these things?
Thanks!
Daren

[Joan R.]

Hi Daren,

Without images is not easy to determinate what mineral or rock are you talking about. It would be much easier with some pictures. Please send some photo/s and perhaps we could try to identify them.

If you are talking about sandstones sometimes there are vugs originated by dissolution of organic materials, with Hematite crusts inside...but we need to see them.

BSRG
Joan

[dgc]

Here's some pictures. The ones I cracked open as a boy are long lost. Some were hollow with a dark lining and some were solid,, smooth gray, flint-like.

[Joan R.]

Hi,
They are similar than river boulders. Sometimes the rounded rocks are embedded or included in limestone or sandstone. Of course, these boulders could have different lithogenic origins. Some could be only limestones (hydrochloric acid effervescence) or Quartz (lidites, silex...) and this materials could show vugs inside.

This is my personal opinion.

BSRG

[dgc]

Anyone else have any info? Thanks!

[Les Presmyk]

Having cracked a few geodes over the years and any spherical rock that folks bring into the show, my first guess would be these are just river rocks. Or, as I tell people, all geodes are round rocks but not all round rocks are geodes. Not very deep, philosophically, but true. Geodes are not this smooth and I believe any that may be hollow or contain something inside are more of an accident than anything.

[Pete Modreski]

Daren,
I agree that these look like they are probably, for the most part, just round "river rocks".

Some of them, perhaps the very spherical ones, could possibly be sandstone concretions; mineral-cemented nodules that form in sandstone. Such concretions can be solid sandstone, or sandstone around some harder central core, or around a fossil, or even hollow inside. Here, I'm posting a photo of a group of such concretions, from near Colorado Springs. Many of these are hollow, which in this case is due to them having once contained centers of pyrite, which has been oxidized and dissolved away, helping to provide the iron that cemented the concretion together. These are hollow, but they are not quite properly "geodes", because they are not lined with crystals inside, just a hollow shell of sandstone. Because such things are composed of fairly soft and friable (crumbly) sandstone, most of them are only found where they have weathered out "in place" from the host sandstone, and, unlike round river rocks which are made of hard quartzite, chert, granite, etc., they do not survive being transported very far by rivers or streams.

Sincerely, Pete

[dgc]

Thanks!

[dgc]

Ive got a new friend with a slab saw - I may slice open a couple of them.

[wardp]

They look like what we used to call Glacier Eggs. You didn't find them in an area that was once covered by glaciers did you?

Just a thought.