Welcome to Petroleum Geology Forums

We have found a very unusual stone. Here's what I can tell you about it. It was found on an old farmstead in Montana in what is known to be an old riverbed. There are many other stones that look just like this one, but this is the only one that has the marking. The square shape appears to be solid stone, so I don't think that the stone had a rusted piece of metal laying on it or something to discolor the surface. Does anyone have any idea what may have caused this? If you have any geologist/archaeologist friends please have them take a look at the pictures. I'm just not sure if this was man made/caused or something natural. Thanks.

Close up.



Overall view.



Side view.



Randy Pepprock
Downtown Deco

I think it's natural crystal growth in this granite. You seem to have found an igneous rock, probably soem sort of granite. Granite solidifies underground and cools slowly. Crystals therefore have a lot of time to form and grow large and in their prefered shape. There are many crystal that have an cubic skeleton, Galenite being a clasic example (http://www.mineralogische-sammlungen.de ... t-engl.htm).

Galenite is not common in granite and this does not look lite it. This may be a quartz pseudomorphs (recrystalised quartz in another minerals space) after a different cubic mineral. Similar to what they describe here: http://mineralzine.chez.com/pages/info2-b.htm