That actually isn't completely true. If you dropped them from a plane at high altitude, the heavier rock, if it was made of the same material, and shaped about the same, would hit sooner as it would reach a higher terminal velocity. If it was made of lighter material, like limestone compared to marble, it might hit later. If it was sculpted into a wide sheet with a slight bowl shape, it might drift off target or shimmy down like a sheet of paper. If one rock was shaped like a spear it would drop faster than one shaped like a snowflake.

If you dropped them from 20 feet, they'd hit pretty much at the same time.