I like to walk a different line here.
I have this conception of dragons-are-angels-are-dragons. Originally, you had two broods and a bunch of non-brood dragons. Dead Io, the Source from which the universe springs, produced many divine children, but Tiamat and Bahamut were the two greatest, and they spawned the Prismatic and Metallic broods. Their divine siblings have their own children, but those dragons are more or less just lesser copies of their parents, not distinct the way the Progenitors' children are. The two were created to be one another's opposite number, the perfect match, which would eventually produce the successor to Io--the dragon that is and contains all things, who will carry the torch on to the universe that comes after.
But something happened, something stole the Egg that was Promised, and Tiamat went mad trying to find it--to the point that she struck Bahamut and fled. Her fall, when she was meant to be Child-Hunter-Ruler (e.g. Son-Hunter-King/Daughter-Huntress-Queen), has broken the world itself. Bahamut, as Sibling-Teacher-Priest, cannot truly rule the cosmos properly; he can guide, inspire, and protect, but he cannot rule the way Tiamat could. And when Tiamat fell, she took her brood with her, creating the fallen Chromatics, so there are no more Ruby dragons, no more Diamond dragons, only Red and White etc. (And none know where the Topaz nor Amethyst went; not even Tiamat, whose shattered mind does not even manifest Yellow nor Purple heads!)
But all their siblings? Pearl and obsidian and opal and tektite, and on and on? They too have their children, albeit few in number, who attend to other aspects of creation.