A Sheen Of Ice On The Tarmac Footpaths...

[sandyRoe]

...but the granite kerbstones didn't seem as slippy. Is it just that the stone offers better grip underfoot?

[Hypognosis]

Part of the idea of tarmac is to get surface water to work its way along the surface cracks and trickle into the nearest soakaway.

Hoar frost condenses directly out of the air, from gas to solid, so it can't trickle away.

So, by 'sheen' are you describing a transparent layer, instead?

If you saw the recent explanations of 'ice storm' on the TV, you'll know by now that this is supercooled water droplets freezing the instant they splat onto a surface. Again, no opportunity to run off, so the individual splashes all freeze together into one ice layer.

All I can say about the granite is that it's as rough as sandpaper, not at all water absorbant but it's not a hydrophobic surface, the way tar is. Droplets lose their surface tension and spread sideways very rapidly ('wetting'). Splash some water on a waxed surface to see the opposite of this. The chemical composition of it may also be such that the quick-freeze behaviour of super-cooled rainrdrops is delayed for a fraction of a second - just long enough for it to spread into a thinner layer, which the gritty surface pokes through when you step on it.

[gen2]

One factor not mentioned above is that tarmac is darker than granite. That means it radiates its heat much faster and gets cold quicker. The granite will therefore have had less time to accumulate its glaze of ice.

[jomifl]

Granite is a better conductor of heat than tarmac(between 3 & 5 times better) so conducts heat up from the underlying soil.