Crystal Skull

In the 20th century, skulls made of crystal were showing up, with people claiming to have discovered them, and that they had Mayan or Aztec origins, and were said to have supernatural powers. Well, it's likely these were hoaxes. Not only were no skulls examined found in a documented excavation, all were dated from no earlier than the 19th century.

But fiction follows Rule of Cool, and the supernatural powers attributed to these skulls are plenty for shows.

Also, despite the skulls actually being made of crystal, works of fiction often depict them as skulls that are simply Gem-Encrusted.

A Sub-Trope of Public Domain Artifact.

Compare Ring of Power, Mask of Power, Flaming Skulls, Crystal Ball, Silver Has Mystic Powers.

Examples of Crystal Skull include:

Film

Indiana Jones and the Kingdom of the Crystal Skull had them as the skulls of extra-dimensional aliens with whole skeletons made of cyrstal.
The Phantom revolved around gathering three mystic skulls carved entirely from precious materials.

Live-Action Television

A crystal skull was found in a giant alien pyramid in Stargate SG-1. It matched one found in an ancient Mayan temple back on Earth. Apparently, Quetzalcoatl was actually a giant alien - no surprise there - and the skulls are a way of contacting him.
Arthur C. Clarke's Mysterious World used a crystal skull in its opening titles.
In an episode of Peep Show, Mark meets a woman whom he actually likes. Too bad for him she fervently believes in the magical powers of crystal skulls. This is a bit much for him.

Music

Doctor Steel had a song called "Curse of the Crystal Skull", about greed for such an object that dooms a pirate.

Tabletop Games

The GURPS 3E supplement Warehouse 23 has an entry on a Crystal Skull. It's a Famously Mundane, Fictionally Magical version of a crystal skull that was found in Real Life in British Honduras in the 1920s. It allows its owner to cast the Divination spell, and it can use all Telepathic powers on its own.

Video Games

The Nancy Drew video game, Legend of the Crystal Skull
Zork has a crystal skull treasure to collect.
Parodied in Kingdom of Loathing with the crystal skull, which is used to scare away monsters and turns out to consist of worthless glass.
La-Mulana has one as a late-game item.
A crystal skull appears in Spelunky as a treasure in Jungle Japes levels. Trying to take one automatically summons a ghost, but bringing it to the exit is worth a lot of money.

Web Comics

In Magick Chicks Dakota, as an Expy of Indiana got one. It's a mind-control artifact.

Western Animation

A crystal skull was a powerful item sought in the first episode of Goldie Gold and Action Jack.
The second season of American Dragon: Jake Long had a continuing plot of collecting the 13 crystal skulls. When they are brought together, one wish is granted to the bearer.

Other

Dan Aykroyd co-founded Crystal Head Vodka, which comes in a bottle shaped like a skull. He's very interested in the legend of the 13 Crystal Skulls.