Mendoza showing the missing piece
The Medallions are the keys to open the Cities of Gold. It has two parts: crescent shaped necklace part and coin shaped piece that can be attached or removed if needed.
Only two of them were shown in the series; Esteban and Zia got one each from their families. Two medallions were needed to open a city. Their origins is roughly explained in the series.
After finding the first City of Gold, the sun pieces of the two Medallions were left in the locks of the first city during it's destruction. However, it's revealed that they were reclaimed by Esteban's father later, who subsequently gave them to Mendoza while kidnapping him in Beijing.
Mendoza exchanged them to Esteban as a sign of proving that he isn't a traitor, while stealing the Pyramid of Mu from Ambrosius, to give it to Esteban's father later.