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We wanted to get some rocks to use as ornaments in our vegetable garden, and the simplest thing seemed to buy a few bags of these "Caribbean Beach Pebbles" from Home Depot. When they were delivered we noticed that the rocks all seem to be coated in a fine white powder, roughly the texture of talcum or chalk dust.

Does anybody know what this powdery stuff is? Is it just the rock itself, or are they coated with something? And is there any reason to be concerned that whatever it is might get into the soil and harm the vegetables in the garden?

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    Could be anything from natural rock dust to powdered pesticide that a store worker spilled on it. Simple fix would be to wash the rocks somewhere where the dumped residue won't affect your veggies... Jun 1, 2021 at 22:33
  • @JimmyFix-it They came in a sealed bag, so the “worker spill” scenario seems unlikely. If it’s natural rock dust, my concern is that even after washing, the whatever-it-is will leech out of the rock and into the soil.
    – mweiss
    Jun 1, 2021 at 22:35

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Probably limestone . The white dust is from rubbing together. They would add calcium and possibly magnesium to your garden. The soil very likely already contains these elements. A drop of vinegar will bubble slightly if it is limestone, muriatic/hydrochloric would bubble vigorously.

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