This is deduced from the vast number of "Transformers" references within the line. There was a rare golden Optimus Prime figure released in 2004 called the Lucky Draw Convoy. There is also an episode of Transformers called "The Golden Lagoon" in which a pool of Electrum is discovered, and any of the bots that enter the pool become immune to being hit.
Appears to point towards some sort of "Electrum" substance, an alloy created from a fusion of Gold and Silver. These was crafted into coins during ancient times.