Copper stairs make stone stairs sound below level 0

I have a copper staircase that goes from the surface to about level -53.  Around level 0 the sound of the stairs changes from the copper sound to the cobble stone sound when running down them.

 

As an aside - I've not seen any minerals or ores below level 0 - is this expected or am I just unlucky?

 

Also the new lush cave blocks are cool.  Thank you for thoes.