For instance iron with a specific heat of 450 doesn't melt, but marble with a specific heat of 800 does, yet adamantine with a score of 7500 is also immune.
The theory isn't just specific heat. It's that PLUS melting point PLUS ambient temperature PLUS dragon rate of fire (and how many dragons, I suppose). Adamantine and iron both have higher melting points than marble.
* The melting point is what you have to get to to melt it.
* The equilibrium point would be the same for all materials in a given situation, in a given ambient temperature, for a given rate of dragonbreath application
* The specific heat would determine the variance (amount of swing back and forth from coolest to hottest in a cycle of breath). High specific heats = less variance, so if the equilibrium point is nearer the melting point, it might survive with a higher specific heat while a lower one wouldn't (because it won't swing high enough right after breath to surpass melting point at the very top of the graph).
You could melt any theoretical material at all with any melting point below 50,000 or whatever dragon breath is. But to do so, you might need multiple dragons synchronized so that every tick is a dragonbreath tick with no time to cool down.
As you go down in melting point from the actual heat of dragonbreath, the need for constant application should fall gradually until at some unknown point, a single dragon on max rate of fire can melt the material.