I must admit I was also a little puzzled at 25 carat and no doubt some logical explanation will be presented before long. You are nearly perfectly right about the 24 carat pure gold but I think it is an impossible number to reach for a reason I cannot remember and 24 carat is actually 23.99r or something like that
You're right - absolutely pure gold is 24 carats, but it's impossible to obtain it economically. The highest commercially obtainable gold is actually 23.988 carats, but it's normally referred to as 24 carat.
In the entire history of the human race 165,000 tons of gold have been mined.