Amulets is an item worn that has apotropaic (magical) properties. They were used to gain favor with the gods, protect against the evil eye ?, increase virility and fertility, protect against sickness and injury, the list goes on.
Amulets have likely been around far longer than we know. Items mistaken for early jewelry were likely some form of amulet. Examples of some amulets are:
The lunula, was worn only by girls until, much like the bulla, they reached adulthood (around the age 12 or 13).
*The Lunula last for quite sometime in history, it is seen during the Viking Age and later.
The Bulla is of Etruscan origins, one of the few unchanged holdovers from the culture. It was worn only by boys (upperclass wore gold, lower class would wear leather). These were worn until the boy became a man.
Worn by boys as both rings and pendants from birth in some cases, these were very powerful amulets meant to protect the boy from sickness, the evil eye, and to protect his virility.
The manus fica is an obscene hand gesture where the thumb is between the index and middle finger. One could assume that since the fascium was to ward off evil than the manus fica with the fascium was telling the evil eye to f*** off. These may have been worn more by soldiers.
*Fascium were also placed on animals and chariots for similar reasons.
**Note that in Ancient Rome, the majority of amulets or apotropaic items are phallic. Masculinity was highly prized in Ancient Rome and was associated with strength. Thus many items represent the strength of the phallus. This can be seen in Greek culture as well (see the book Reign of the Phallus)