What is "Pure Gold" ? and what is meaning of " carat" with related to the purity of gold ?

Purity of ornamental gold is expressed in term of the "carat" and "24 carat gold" is 100%  pure on carat scale .

We know that 100%  gold is very soft so that not suitable for making ornaments then increases hardness of gond by adding silver or copper to make ornaments. Thus find the percentage of gold is "x" carat gold the % of gold and copper calculated as:

% of of gold in "20 carat gold" = 20/24×100= 83.33%

Melals like Cu, Ag, Zn, Hg and Pb occur in nature as sulphide rather than oxides. Why ?

The cations of these metals i.e. Cu+2, Ag+, Zn+2, Hg+2 (pseudo inert gas configuration), and Pb+2 ( inert gas configuration) have high polarising power and hence can easily polarise the bigger S-2 ion than the smaller O-2 ion. Hence sulphides are more stable than oxides of these metals.

Cu+2, Ag+, Zn+2 and Hg+2 are soft Lewis acids and S-2 is soft Lewis base . Compounds obtained by reaction between soft Lewis acid and soft Lewis base is stable. O-2 is hard Lewis base . Adduct of soft Lewis acid and hard Lewis base is not very stable compounds.