Excerpt from the Aristo’s Guide to Arcadia Geology 455 Ab Terra Condita:
Gaea stones are a naturally occurring Arcadian crystals similar to quartz found on ancient Earth. Hephaestian scientists discovered Gaea stones have many unusual physical properties including their ability to hold an electrical charge. Last century the Hephaestus sect developed a process to charge the Gaea stones for use as batteries in all electronic devices on Arcadia. Today they are the sole source of energy on Arcadia replacing all previous storage devices brought from Earth.
The standard Gaea stone will run an electronic device for two to three months before having to be replaced. Larger crystals can even be used to power vehicles such as a Pegasus or one of the Hermian Caravan ships. The only known source of Gaea stones are in the mines of the Vulcan Mountains near Hephaestus’ capital, Erebus.