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.
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.
"Gold staters" vary in gold content a great deal:
and https://en.wikipedia.org/wiki/Celtic_currency_of_BritainPure 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.