1. First of all, what is gold ? 

Gold results of nuclear fusions in the stars. There is thus a great quantity of gold in the core of Earth. In a plus, probably, its presence on the earth's crust is due to the impacts of meteorites and comets there are 4 billion years.
Gold is a metal very malleable, soft. So, to make objects in gold, we must mix it with other metals. All this brings us to the definition of the carat.

 

2. The carat

 

The carat is the unity of the quantity of gold in relation to other metals.

For that, we divide the alloy in 24. So, 24K gold is pure gold. But, it can't be. It contains at least 99,99% of gold. 18K gold contains 75% (18/24) of gold and 25% of other metals. These other metals give the colour of gold.

For 18K gold :

- Yellow gold : 75% gold + 12.5% silver + 12.5% copper

- Darker yellow gold : 75% gold + 10% silver + 15% copper

- Pink gold : 75% gold + 5% silver + 20% copper

- Rose gold : 75% gold + 2.75 silver + 22.25% copper

- Grey gold : 75% gold + 15% silver + 10% copper

- White gold : grey gold covered with rhodium

Before, we could find gold with nickel but now, it is prohibited in Europe.

 

3. The mint marks

All objects in gold must have a mint mark with the origin of the object : a diamond for gold, a square for plated gold.
All objects in gold of more than 3 g must have a mint mark with the carats (guarantee of the State.)

- 9 carats : a clover or 375

- 14 carats : a shell or 585

- 18 carats : an eagle or 750

- 22 carats : an eagle with 1

- 24 carats : a seahorse or 999

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