The phenotype is just a characteristic/trait.
For instance Brown and Green are phenotypes of eye color.
This can also be used in Punnet Squares, and could be displayed as B and G.
B equaling dominant Brown, and G = dominant Green, vice versa to g and b where they are recessive.
It is because it's surface area is responsible for the production of ATP molecules.
I would think that an unweathered rock would be jagged while a weathered rock would be smooth because of water/wind or what ever weathered it.