Shiny metallic coating on rocks, Iron?

[OwenT]

I was in an area where I kept seeing rocks with this shiny metallic stuff on the outside. It seems to be just a coating, usually covers about half the rock. What is it?

 

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[DDancer]

Where are you seeing these coated rocks?

 

[OwenT]

Above Douglas Creek WA. Specifically right over the spot where there's an old caved in railroad tunnel so I wonder if maybe some of the rocks were brought in for fill because I'm not sure if they're from around there.

 

[DDancer]

Good possibility that they are fill rock. The coating appears to be similar to iron oxide like hematite and may be present due to chemical interactions with the environment and some form of extreme heat. I usually see stones like that in the desert southwest, thats why I asked where you see them, that have a similar patina on the sunny side of the stone. Such rocks have a high iron content and in the desert the extreme heat and occasional moisture create the affect your picture shows. It can be pretty neat to pick one of these shiny black rocks up to find a lily white bottom.

 

[Eu_citzen]

I sometimes see the same coating in rivers around here. Never thought of what they where.

 

[OwenT]

Good possibility that they are fill rock. The coating appears to be similar to iron oxide like hematite and may be present due to chemical interactions with the environment and some form of extreme heat. I usually see stones like that in the desert southwest, thats why I asked where you see them, that have a similar patina on the sunny side of the stone. Such rocks have a high iron content and in the desert the extreme heat and occasional moisture create the affect your picture shows. It can be pretty neat to pick one of these shiny black rocks up to find a lily white bottom.

That's got to be it. Seemed like there was lots of iron in the rocks, definitely exposed to high heat and occasional moisture, and the effect seemed to only be present on the sun-affected side. Thanks