I must've missed the part about the H/P stone being granite? Where was this info from?
The only thing I've seen on it was the DAI geologist who said:
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I've heard the tape from Jim Hatt, where Doyle Harnish describes the Latin Heart as black slate. It also sounds like the heart stone was red quartzite, but can't really make it out.
EDIT: I'm not sure what she means by Coconino sandstone, but I'm assuming she thinks it originates from a geologic formation i.e. the Coconino Plateau.
PS - My wife has been a geologist for over 30 years and says there's no way to precisely date rocks. If they're in place where they formed one can make assumptions about age in terms of geologic strata/epochs and any associated weathering that correlates with assumptions about ancient weather patterns. This is very basic, such as associating fossilized ferns etc. in strata with such plants needing a temperate/rainy environment. Or obvious signs of wet vs. dry weathering.
If they're not "in situ", some assumptions might be made about signs of weathering on stones, or lichen formation, but it's a WAG and there's not much certainty that would be provable using established scientific method.
My take-away is that accurately dating stones to within a hundred years is pretty much a guess, and probably based on weathering.
There are newer technologies being used today but they weren't available in the 1960's or 70's.