[defrost]

Gold is a noble metal - pure gold doesn't tarnish like that in your photos.

"Gold staters" vary in gold content a great deal:

    The Durotriges issued a series of rapidly debased coins through this period probably starting around 50BC with a largely silver (80%) stater (British B) with a fairly small percentage of gold.

and

    Verica's stater series weighed between 5.27g and 5.29g while the gold content varied between 42% and 44.5% The gold content appears to have remained stable over time with no sign of debasement.

https://en.wikipedia.org/wiki/Celtic_currency_of_Britain

Pure gold nuggets unearthed after many thousands of years under ground "look like gold" when given a good rinse to get the dirt off - they don't look tarnished like the "gold staters" in you photo.