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.

enter image description here

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.

enter image description here

  • This might be better on earthscience.stackexchange.com – paparazzo Oct 12 '16 at 12:32
  • 3
    I've seen something kind of similar before but it was under an oak tree. Is it possible that a previous owner sprayed something on the rocks to make them "prettier" and the plants caught some over spray? It is strange the rock has a uniform coating but the plant doesn't. – Erik Oct 12 '16 at 15:04
up vote 13 down vote accepted

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]

enter image description here

1 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1392883/

  • Desert varnish is neither wet nor oily, but OP accepted it, so, maybe their rocks were not really wet nor oily. – Beanluc Sep 15 at 0:17

It could be a sugary secretion that some insects produce and that has fallen from the trees. It is usually called "honey-dew".

  • 1
    Is this known to drop onto objects and cause the characteristics that OP describes? – Aaron Sep 14 at 14:47
  • I'd say it is a possibility, from what happened to my car and several rhododendrons this spring. Except that there doesn't seem to be a tree over the rocks, from the OP's photo. – ab2 Sep 14 at 19:34

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.

  • 1
    I fail to see why one answer has been accepted and upvoted heavily while this one is heavily downvoted, all in the absence of any indication that OP has successfully ruled out this answer and proven the other. And someone Erik said something similar as an answer-in-comment, yet that has 3 up-votes. Second: Even if OP did prove that the reason was "desert varnish", that doesn't rule out this answer as being a correct answer in general (ie: for other users). Rock painting is a real hobby, and I have seen rocks covered in this manner, even half-covered as paparazzo suggests. (+1) – Aaron Sep 14 at 14:45

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