Why are diamonds so valuable? Because people say they are.
Sure, sure...you can discuss the difficulty of acquiring diamonds compared to copper or quartz, but the only reason anyone goes to the trouble (beyond industrial uses like drill bits) is because someone somewhere decided that this particular mineral was more beautiful-desirable-valuable than another.
What we deem valuable makes a difference.
Larry Norman's song Diamonds touches on the difference between intrinsic value and conferred value. It's all of 98 seconds long (and really, is more of an introduction to the next song on the album than an actual song of its own), but still packs a wallop.
You can take your diamonds and lock them in your room
and you will lose everything you save
You can take your flowers and lay them on His tomb
but I know He rose up from the grave
* * * * * * *
Truth is...What good is it for someone to gain the whole world, yet forfeit their soul? Or what can anyone give in exchange for their soul? (Mark 8:36-37 NIV)