Ruby crystal

Ruby crystals are highly coruscant gems that can be found on Earth, specifically in the ruby crystal mines of Burma. Transformers, or at least Decepticons, use them as an energy source.

More than Meets the Eye
Needing more energy to make the trip back to Cybertron and conquer it in the name of the Decepticons, Megatron ordered his troops to head to Burma and dig up ruby crystals. The amounts they found were such that Megatron believed they could power their entire home planet with them. Of course, the Autobots couldn't let this happen and blew the mines up. The Decepticons still managed to get away with some concentrated energon cubes filled with ruby crystal energy.