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.

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Shiny part on right (that was exposed to the air), dull part on left that was in the dirt.

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Not sure if it’s the same stuff, but this residue is on the plants in the affected area.

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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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