I found it hard to assess the difference. According to a quarryman, the dust I bought was not dusty enough – it needs to be fine unless you are spreading it for your grandchildrens’ benefit. It almost certainly does some good, probably increases the mineral content, especially in poor soil, but needs soil life from compost etc. to help unlock its goodies.
I had more striking results from lava dust but it is hard to buy as British stockists find it unknown here – I was sent a sample from Osmo in Belgium (a different Osmo I think to the paint manufacturers). All slightly frustrating!
But maybe others have good stories about using rockdust?