What are carbon spots? Good Read!
Disclaimer This is NOT my work!!!
"Spots on coins are another example of bad surface preservation. The coins that are most susceptible to spotting are cents which are made of copper. Copper is a highly reactive metal and it is not unusual to see “carbon spots” on them. They are called carbon spots because many times they are formed from carbon in the air or mishandling by people. Once a carbon spot is in a coin it is there forever and will leave a pit that actually grows deeper over time.
"Even though copper coins are the most common coins to have spots, gold coins sometimes have spots due to the fact that copper was used as an alloy and if there is an area in the coin where the alloys weren’t evenly mixed, a spot can form where there is a concentration of copper."
Here are 2 good links as well: