Contact between rocks over long periods of time causes rocks to smooth.
Type of rocks salt, quartz and basalt are decent types of aquarium rocks.
Contact between quartz over long periods of time causes it to smooth.
what needs contact to smooth over long periods of time?

[1] oxygen
[2] soil
[3] an object
[4] h2o
[5] rivers
[6] quartz
[7] cartilage
[8] bamboo