Some people believe that "once a rock, always that rock".  But that is not always true.  Rocks take different forms at different times.  A long time ago our earth was very volcanic.  As these volcanoes cooled and vast oceans swept over the earth, the cooled lava was broken or crushed into small pieces.  These small pieces were cemented together to become sedimentary rocks.  These rocks were buried and the heat and pressure changed them into metamorphic rocks.  They might even have melted and become igneous rocks once more.  As you can tell a rock may change many times and the rock you hold today may look entirely different to someone a long time from now.