What is this wet/oily residue all over the rocks in my yard?
I live in the Sierra Nevadas, a little over 8,000 feet up. There's an area in my yard that has rocks with a bright shiny sheen to them. I've seen some kind of coating on nearby plants (which doesn't seem to harm them), which I'm guessing is maybe the same thing.
Some facts:
- It's a ~30 square foot area.
- Rain, shine, cold, hot: it always looks the same.
- It's not wet to the touch and doesn't rub off easily.
- If I flip a rock over (shown), the part in the dirt doesn't have the sheen.
- It doesn't really make the rocks very slippery; not anymore than normal.
My question: what the heck is on my rocks (and plants)? I've attached a picture looking up. There's a regular douglas fir and a residential power line, but there are powerlines all over the neighborhood and douglas firs all over the yard: this doesn't show up anywhere else. And a big area of the sheen just has blue sky overhead.
Example shiny sheen rock. The bottom of a nearby rock laid up against it.
Shiny part on right (that was exposed to the air), dull part on left that was in the dirt.
Not sure if it's the same stuff, but this residue is on the plants in the affected area.
I went into the middle of the area and snapped a picture looking straight up. Residential powerline overhead (~30' up), standard issue douglas firs nearby.
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3 answers
Desert Varnish
There's a Wikipedia article on it right here: https://en.wikipedia.org/wiki/Desert_varnish
In a nutshell, Desert varnish (also known as rock varnish) is a dark, thin (usually 5 to 500 μm thick), layered veneer composed of clay minerals cemented together by oxides and hydroxides of manganese and iron.[1]
1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1392883/
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Sure looks like a high gloss sealant to me.
Clean the rock up and apply a high gloss concrete/masonry sealant to the other side and I bet you get a match.
Or take a wire brush to they shiny side.
Hard to believe it is a natural occurrence when the rock next to it has no shine.
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It could be a sugary secretion that some insects produce and that has fallen from the trees. It is usually called "honey-dew".
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