There have been some beautiful and ancient crystal skulls discovered over time that have been certified as being from Aztec and Mayan cultures. One of the main ways they date these skulls is by examining in minute detail on the crystal surface as to how they were carved. By examining these on a micro level they can date the skull.
Some believe avidly that there are some skulls that have been left by extra-terrestrials containing information for us to decode. Also that meditating and communicating with the skulls both ancient or modern can reveal ancient and divine information. So if you wish to have around you or wear a crystal skull this is a symbol of connection with ancient cultures and also a connection to information held deep within.