Complex sentence: Most metals that are of similar or higher density than gold are similarly or more expensive, and were unknown in ancient times (notably the platinum group); only two relatively inexpensive substances are of similar density to gold: depleted uranium and tungsten.
Split into simpler sentences: Most metals that are of similar or higher density than gold are similarly or more expensive, and were unknown in ancient times (notably the platinum group). Only two relatively inexpensive substances are of similar density to gold: depleted uranium and tungsten.