Energy Crystals are a powerful fuel source that typically appear in places rather inhospitable to most forms of life.
Energy Crystal come in numerous variants, natural crystals are formed on world that have been bombarded by cosmic radiation for thousands, millions of years. The radiation has over time changed the molecular structure of the stone/minerals, creating crystals which store the energy making them radioactive. Combined with their own radioactive nature and their environments even Transmetal and general Super Robotic life forms would have trouble mining them. Even droids and other machines would find their electronics fried of they stayed near them too long.
Races that know how to use them would find them to be among the cleanest most efficient forms of fuel. In many cases Energy Crystals are explosive.