Rocks became a collectible because thousands of people think they are cool for different reasons.
people liked it
Hold on to them for a while or have the creator sign it
Yes.
Erosion and weathering will cause rocks to become rounded.
By weathering
No. It's the Metamorphic rocks that melt and become magma.
Erosion causes them to become smooth from wear.
now they are rocks before there werent rocks
Metamorphic rocks become igneous rocks by melting from heat and pressure, remnants of which may cool from melt (magma) and become components of igneous rock.
Two of the most common fates of igneous rocks are to become sediment (and later sedimentary rocks) or to become metamorphic rocks.
Sedimentary rocks can be eroded into sediments and form sedimentary rocks again, they can melt and become igneous rocks, and they can undergo extreme heat and pressure and become metamorphic rocks.
All rocks can become igneous rocks. With weathering and erosion, an igneous rock can become a sediment. Then with compaction and cementation, it can become a sedimentary rock. With heat and pressure, it will turn into a metamorphic rock. Or it can melt and turn into an igneous rock. There are many more ways rocks can change types... that is the rock cycle.