It is true that the hardness of the gold does depend on how you alloy.
In general..the higher the karat of gold, the cleaner and softer that it is
(again, allowing for different alloy mixtures of course).
Using only gold, silver and copper, with 18k alloys generally
the ones with more copper are harder, with 14k the ones with
about 20% silver are the hardest. An 18k alloy with 5% silver
has a hardness of about 140 HB (Brinell), a 14k alloy with 5%
silver is about 105 HB. The hardest 14k alloys are about 160 HB,
the hardest 10k alloys are about 190 HB, one with 5% silver
about 95 HB. All annealed and quenched. You can, however, harden
all the alloys, an 18k copper-only alloy to over 285 HB by
tempering it at 250 C - 400 C for several mminutes, whereas a
14k alloy will not get harder than 280 HB. 10k alloys can be
hardened to more than 310 HB.
In Europe, 10k gold is not used, but there is much "jewellery"
made in 8k "gold", and its really a headache to work with that