Fine gold or pure gold is by definition 24 carats.
Spot price is for pure gold so when figuring out the value of your gold coins or jewellery you need to determine the actual gold content in calculating price or value.
Sovereigns for example are 22 carat so the real fine gold content is 22/24 ( 91.66% ) of the weight.
Jewellery stamped 18ct is 18/24 = 75% gold and 25% other metals like silver or copper.
Rings stamped 9ct are only 9/24 = 37.5% by weight gold and this is why you may also see the figure 375 stamped.