What is a Patina?

By GovMint :

A patina refers to the natural surface layer that forms on coins, particularly those made of bronze and copper, over time due to oxidation and environmental exposure. This layer results in vivid colorization that often appears as shades of green, blue, red, or brown. Rather than being a negative, coins with patinas are prized by some collectors for their unique aesthetic appeal and as a physical sign of a coin's age. However, collectors must be cautious of artificially induced patinas, which are sometimes applied to mimic the appearance of genuine aging.

Sources Used:

https://coinweek.com/patina-on-ancient-bronze-coins/