I urge them to look closely, to get down on hands and knees for a few minutes, use their magnifying glasses, and then tell me what they see.
By geological classification, these rocks are 'metamorphic', meaning that they have been transformed under punishing heat and pressure beneath the surface - and then, astonishingly, come back up.
Unlike an igneous basalt crystallised from lava, or a sedimentary sandstone laid down by water, metamorphic rocks form in one environment, then go on journeys deep in the crust.
This makes them the itinerant 'travel writers' of the rock world, returning to tell us about the restless, animate, hidden nature of the solid Earth.
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