Specifically, the crystal skulls have been shown to be a fake:

Humans seem to have a predilection for fake quartz-crystal Aztec skulls. Since the 1860s, dozens of skull sculptures have appeared on the art market purporting to be pre-Columbian artifacts from Mesoamerica, that is, created by the indigenous peoples of Mexico and Central America prior to Spanish exploration and conquest in the 16th century. Three such skulls have graced the collections of major museums on both sides of the Atlantic: the Smithsonian Institution in Washington, D.C., the British Museum in London, and the Quai Branly Museum in Paris.

………

So the team took a closer look at the skulls’ surfaces. As a benchmark, they borrowed a legitimate Mesoamerican crystal goblet from the Museum of Oaxacan Cultures, in Mexico. Then they used scanning electron microscopy to compare these surfaces.

It turns out that the surface of the authentic goblet has irregular etch marks, a sign that the pieces were carved with hand-held tools. But the surface of the suspect skulls have regular etch marks, evidence that they were made with rotary wheels and hard abrasives, which appeared only after the Spanish conquest of Mexico, Walsh says.

Looking even closer at the British Museum’s skull, the team discovered green, wormlike inclusions in the rock. Raman spectroscopy revealed that the inclusions were an iron-rich chlorite mineral. Although this kind of trace impurity is found in rock crystal from Brazil or Madagascar, it is not found in Mexican crystal, Walsh says.

Note the Difference between the surfaces of thehand carved goblet (r) and the skull

Not surprised.

When you find artifacts that create this kind of “Ancient Astronauts” type hysteria, they are almost always bullsh%$.