Actually, from what I've read of the
original accounts they're said to be harder than steel.
What ancient legends do refer to is dragons accumulating a coating of precious gems on their underbellies. There is no known natural process to produce harder-than-diamond materials, although some artificial means have been discovered (Dragonrider, you're going to like this one):
http://www.mcclatchydc.com/256/story/71495.htmlAnd just out of curiosity, what dragon sites are you referencing? I don't want a long list, just a few examples, cause I'm having trouble finding fact-based dragonology sites other than this one.
This site's take on scales:
http://www.dragnix.net/From_tail_to_snout/D-Wars: Dragon Wars is a movie:
http://en.wikipedia.org/wiki/D-War The first and only place I've seen dragon scales compared to diamonds.
Interestingly enough, the dragons there are susceptible to the miniguns on Apache helicopters, hinting that only the Buraki (giant serpent) had harder-than-diamond scales, and the Bukra (dragons) had inferior scales.
As to your questions, I agree that they are good ones, but most are directly involved with what the scales are made of. For example, the material would determine their weight, and subsequently whether flight was possible. In the book “Dragon Rider” by Cornelia Funke, the golden dragon has impenetrable, harder-than-diamond scales, and as a result is too heavy to fly. Also, minerals would be harder to replace than organically grown compounds, so mineral-based scales are less likely to be wantonly discarded during growth.