I recall a science-fiction story that started with "There are no millionaires on Blarg." This turns out to be because they don't use gold for coinage, but uranium. The millionth coin makes your money-pit go critical.
Maybe your planets gold has a uranium/radium/plutonium impurity?
"Daddy, why is the dragon's hoard glowing?"
"Dragon gold is magic, everyone knows that."
"Oh. Daddy, where did all your hair go?"
"Err.. dragon curse maybe? Now hand daddy his new dentures."
This, of course, implies dragons are powered by the radiation given off by their hoards. Which explains a lot, frankly.