Honestly? I live in a sedimentary pseudo-basin, so to me the most exciting things are the building stones. Which -- don't get me wrong! There's some lovely ones! There's a garnetiferous marble used to face this one building, which has streaks of tiny garnets pressed into waves and curls and it's great. There are also My Favourite Kerbstones, full of great big semi-aligned plagioclase laths; and lots of nice labradorite granites.
Much more interesting is the coast around the Mouldering Ancestral Pile: the beach a five minute walk away (Polurrian) features
an outcropping of the Lizard Boundary Fault - the join between continental and oceanic crust. And then there's the Lizard complex as a whole, wherein what-used-to-be-the-mantle is actually exposed at the surface - over a few miles of coast you can actually walk from the base of old oceanic crust up to the surface, through gabbroic cumulates and sheeted dykes and pillow basalts; you can literally stand on an exposure of the Moho-that-was, the boundary between crust and mantle, and it's
really cool. SO: for these purposes, Cornwall is home much more than London is. ;)