Gold is gold and it is as hard as it is.
Studies have shown that all things being equal, the gold content of an alloy does not seem to affect its durability. What you are really talking about are the alloys used in the karated gold. The three most abundant alloys we find in karated metals are silver, copper and nickel. There are other metals in small and even trace amounts, too. Among the three, nickel is the toughest and also the hardest, followed by copper, then silver.