I think it’s mentioned in II:24, dragons by volume eat a lot of stone. Their scales are woven silicates (green dragons are different). What was not mentioned is that a dragons environment has as much to do with its color as any heredity possibly could. Particularly greater dragons, who have ascended from mortal form. The environment of transformation will predispose the color of scales.
Black dragons originate from the black granite quarries north of the Osyraen capitol. This was where Prince Vhale experimented on his subjects, before becoming the Black Emperor Vhale’raus. A name taken to brag that he had taken the Sun, as the light barer he is named for, never could. With the exception of Mar’etten, and Lestra of Thebes all black dragons live in the Osyraen mountains. Lestra’sa herself has shown with her gray offspring the results may vary with the heritability of dragon coloring.
The white flight are the children of Roshana who herself transformed inside a white granite tower. Though the stone of Napir is relatively pale, it is not the white of central Corinthia. None the less it has not proven overpowering to the coloring of her children, not even Calista’etten, who was dragonborn, and ascended in her late nineties in Napir.
The green flight are all children of Alara, and are as a mater of fact photosynthetic. Alaram and Roshana’s mate both ascended amidst forest. Do not however permit this more organic, leaflike substance fool you. Green scales are actually harder due to complex carbon constructs under their photosynthetic exterior.
What does become curious is that the White Flight is white, and the Green Flight are green, while the sire of these broods are a green dragon, and a black. Clearly demonstrating the importance of the mother in dragon heredity. Though it is highly probable the reason for this is the magical part of the offspring comes almost entirely from the mother. Dragons have human (dragonborn) children primarily because they genetically are human. The rest is a magical overlay, a parallel pedigree of arcane origin. At least for the first generation, and to some extent the second, third, and sometimes fourth. Eventually however the spell rewrites what lays under it. This leads to draconic descent (falling) of the generations. Becoming more beastly.
That is until the seventh generation. Which currently present a cap on dragon population through reproduction. Seventh generation dragons cannot reproduce. Most of their eggs come out fossilized. Examination of the interior of the stone shows structures like a dragon whelp in different densities of stone. Some are strange inert collections of inorganic material. Seventh generation dragons are fairly docile, and eat almost entirely stone. The gray brood of Lestra cap out around three generations. This is believed to be a consequence of her having been turned against her own will, and having been (largely) ungifted servant in mortal life.
Lestra is as such one of the chief chinks in the assertion that the gift cannot be given. As while she is quite pale compared to other ascended dragons, she is still quite powerful in gift compared to most mages, or other practitioners. While most dragons can weave spells into their breath to produce desired effects, Lestra has difficulty doing so. The natural manifestation of it however is curious, and quite effective. It does not freeze in a conventional sense so much as attempt to crystalize what it touches. Her reduced gift means that she can manifest far less of this power than other dragons. Attempts to replicate the effect by mages have all failed. None of her offspring have shown any similar capacity.