Rocks are cool, I don't understand why anyone wouldn't appreciate a collection of them.
My mother is trying to downsize her stuff because her health is going downhill, she gave me the best of her rock collection and it's on the shelf next to me while I type this ... they're really nice rocks.
Hmm ... niche ...
One I got from a friend of a friend is a taste in shitty old cars. Whenever I'm out walking the dogs and happen across an obscure or currently rare, but shit, car I'll take a photo of it.
Everyone loves the "halo" models, the GTIs and Turbos, but I love to see a rusty 1.4 diesel that didn't even come with carpets when new :-)
I've never ever found another woman to nerd out about old cars with, unfortunately, and most guys do the knowledge test thing which is most offputting :-/