rock dust

I live next to a granite quarry and when they crush the stone they end up with a dust that is as fine as talc, Can this powder be useful? Has minerals in it but I am not sure of the best usage. Quarry man said the only person he had seen use it would put a spoonful in a beverage every day.
fsink

Rock dust is actually a fairly popular gardening amendment right now. It's supposed to be a good way to incorporate a wide range of trace minerals into the soil so they can be taken up by your plants. Basically like a multivitamin for the garden. It's supposed to help the plants be healthier when growing and more nutritious when eaten. I wouldn't turn down some granite dust to add to my limestone based soils, but am not interested enough to spend money for it.

Granite is potentially high in radioactive materials, and granite is much higher in radioactives than other types of rocks are in general. The radioactivity can be significant, and bad enough that there have been lawsuits regarding granite countertops in some cases. So unless you're screening your granite dust with a geiger counter and know what you're doing I would advise against it.

Also granite is generally low in nutrients, and most rock dust used for agricultural purposes is basalt-based. Even then rock dust isn't particularly efficient as a fertilizer, unless you have no soil at all, or pure quartz sand, or pure peat, or something like that. Or if the rockdust happens to be free and nearby.

Disclaimers aside, there's pretty much no limit to how much you can use. You can grow plants in pure rock dust, limited only by how well it holds water and how much nitrogen is available.

If you live that close, I would imagine that your land has received a light dusting of that dust repeatedly over the years.  Any nutrients that might be available in that dust is most likely already present in your soil due to the drift of that dust over the years.  So while you might want to incorporate some into your soil, it's probably unnecessary.

I wouldn't be straight-up eating the stuff --- particularly by the spoonful.  Beyond sounding really unpleasant and gritty, I'm not sure what that would contribute to your body.  People weren't created to eat rocks, powdered or otherwise.  Anything you body needs should be readily available in a broad selection of healthy food.