I asked my dad which gold coins were the oldest in his collection. He showed me two. Both from the 18th century.
Empress Maria Theresa, Hungary, 2 ducats 1765, Kremnica mint.
Coin diameter 25 mm, 6.98 g of gold 986/1000.
Emperor Joseph II, Lombardy, Souverain d'or 1787, Wiena mint.
Coin diameter 22.4 mm, 5.53 g of gold 919/1000.
Although the coins are gold, their historical value is said to be greater than the price of the metal. But if worst comes to worst... Gold is still gold.
But I hope we never have to sell those coins. Dad also has quite a few newer gold coins.
I apologize for my poor English.