Rusting skin on weathered rock

[Ron_Noco]

Hi all,
I found this rock on the seashore, reason it stood out is, it looked like a large rusting egg,
It’s a well weathered rounded rock, i ground a small window at first to see what was underneath, nothing special so I split it in two, I was hoping it was more exciting internally but alas,
Anyway the rust on the outside still intrigued me so I did a little experiment, I put one half outdoors for a few months and kept the other warm and dry inside, as you can see in the pictures the rust is bleeding off the exterior and no rust has formed on the shear?
Rock not magnetic, but the rust looks like a skin burned on
Any ideas?

Thanks

 

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[DDancer]

What you have is a concretion that contains iron minerals. The red is iron oxide that is leaching from the exposed minerals in the concretion. Interesting that you made an experiment of it. In the photo it looks like pyrite is present at the very least, the silver bits. Before you split it the outer "rind" oxide layer protected the inner portions from the environment and oxidation so by leaving one half exposed and one half protected inside you have a nice display of what occurs to that mineral in nature.

 

[Ron_Noco]

Hi there, thanks for that, secretly your always hoping to have found something special, one of these days

Cheers

 

[IAMZIM]

Hi there, thanks for that, secretly your always hoping to have found something special, one of these days

Cheers

Always keep your eyes open and I'm confident you will!

 

[DDancer]

Something special is like beauty, its all in the eye of the beholder. I'd say you have something special there.