I've always contemplated the use of those small granite/concrete/whatever rock pieces you see on the road in Singapore, could those be used for tying on nanas/Java ferns?

What I do now is bury the wood chunk that came with the Java ferns, and it's kind of clunky to do so. Anyone knows what material those rocks are made of? You know... those small pieces of pebbles you find at the roadside, those that you kick around. My main concern is the inertness of those rocks.

If they are granite, I'll pick some home, give them a vinegar test and if all is well I'll be experimenting with the possible use for them.