Back in the late 2000's I worked with a bunch of scientists testing granite countertop materials for radiation hazards. Some types and some slabs or granite were crazy radioactive, the record slab found was hundreds and hundreds times hotter than normal background radiation levels and there were plenty of types and slabs in local slab yards that were five, six, even ten times background radiation levels. Studies were done on the radon gas and there were homes found that had high levels of radon gas due to the nuclear decay chain in the granite and after removal the levels dropped back to normal for the area. For the most part granite turned out reasonably alright if you were lucky and didn't get a super hot slab installed in your kitchen.
But, as we were studying this issue we were using email threads with two or three dozen scientists contributing and one dude was a uranium geologist who mentioned that Uranium wasn't the only heavy metal present in granite.
Fast forward about nine months, a three ton shipment of granite samples owned by Silestone had been sent to Israel for testing and the deal fell through on the testing so Silestone shipped the samples to me. I loaded about a ton of them into a truck and ran them up to a geologist in Colorado Springs CO and had them XRF gun tested. Holy moly, those granite samples were loaded with all sorts of heavy metals including massive amounts of lead. Cadium, chromium, arsenic, all of the radioactive decay chain products which was to be expected.
Many heavy metals aren't "bio available" in their native state. Like Iron, not much iron is found in nature as pure iron because oxygen eats it up and coverts it into rust. People read sci fi stories about an exotic planet with a corrosive atmosphere, well that would be earth believe it or not. Oxygen is highly corrosive. So where a heavy metal has been exposed in an ore (chicken grit is an example) the heavy metal might be oxidized enough to be unavailable due to the oxidation. Or a surface layer has been oxidized and the underlying element is still quite toxic and bio available. You just never know.
What we learned was that many things can reverse the oxidation. Moisture, oil from a pair of human hands or skin, cleaning products like soap, changes in the alkaline state (changes in pH), but ingestion was the quickest and most sure change factor. Stomachs have acid so the pH changes rapidly and severely and that is why little Billy eating paint chips is not a good thing.
Same with your chickens. The grit you purchase or that the birds pick up is far more likely to have lead and other toxic heavy metals than the oyster shell you feed them. Pollution is solved for the most part by dilution, the heavy metals will concentrate in something that eats a lot of toxic elements which is why predator species usually accumulate more toxic heavy metals than the prey species that pick up the toxins from vegetation or eating soil for minerals.
If you are worried about lead, go buy a lead test kit from a big box store or order a few online. Test the grit, oyster shell, or dirt and while not completely reliable it will show a result if you have mass contamination.
And don't put food on granite countertops even if you recently cleaned the top! The stuff is bacteria haven too with bacterial living inside the granite, something found a mile below the surface of the earth and from samples taken in biological secure fashion that prevented contamination. Granite is teeming with bacteria and most people aren't aware of that.
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