As for the material, I was under the impression that power cables are not copper either, or that they had to be for proper earthing.
They aren't always. Overhead power cables tend to be aluminium wherever you are simply because it is much lighter but also at such a high voltage that it matters less. Domestic wiring should
be copper. The US went through a terrible phase where aluminium was used as being cheaper. This was a bad idea. In the UK we instead redefined how wiring was done to use ring circuits, the original reason being to reduce Britain's dependence on imported copper in the event of future conflict, a decision taken at the end of WW2 when much reconstruction was required.
An example of what happens when earthing is not entirely copper is the experience of a former colleague of mine in the US:
He'd moved house and was having lots of problems with fuses blowing, computers having faults, bad TV and radio reception, slow modem speed, all kinds of stuff really. He checked the wiring and found that the plumbing was being used as the earth. He then traced the copper piping and found in his basement that the water inlet passed through a galvanised tank. Just to test things he hooked a length of copper wire between the inlet and outlet copper pipes on the tank. The first he knew that anything was better was one of his sons shouting "Dad, what did you do?!? The TV reception is amazing!". Problem solved.
He then contacted a local building inspector to inform them of this seriously substandard installation, demonstrated the fault and was surprised at their response that that was in fact an acceptable way to build a house in the US.
I live on the ground floor. Could I stick a copper stick in the ground and wire it to my PC?
Well, you could do as a suplemental earth but you would then have to ensure good continuity to earth, make sure it is wired up to standard and ideally make sure there is no connection between your supplemental earth and the existing earth. Better still, sort out the earth in the building. You simply shouldn't have unearthed sockets in this day and age.