Not by any means a geologist, but had been told many years ago that it totally depends upon the type of rock dust being considered. Apparently some of the harder rocks would take a long, long time before their various mineral components would weather and break down enough to be rendered usable for any plant use other than providing any benefit other than drainage and loosening of the soil. Even the softer ones have to be almost like a powder and even those may take a year or so.
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