Precious-Opal and/vs X-Ray Diffraction

(My (old) background is math & computer science. I've long loved (mostly) precious opals - for thier esthetics, lapidary potential, synthesis (which I've done), and how the play-of-color ("PoC") is produced.)

I recently did another couple of passes on the latter in the process of trying to figure out how the contra-luz flavor of PoC is produced.
(Contra-luz is transmission PoC rather than reflection PoC.)

I was trying to do the research efficiently, which came down to decisions on how much I had to go back to learning ... optics? ... from scratch all the way up to what I need for this effort -vs- trying to understand as much of it as I needed to just by trying to make more & more sense out of material that's intended for people who have such a background.


One of the places I happened into is X-Ray ~optics~.

But it was hard to make the correlations betw/ this range of wavelengths and the visible (for opals).
...even though, put that way, it sounds like it should be trivial.

Problem was that uses of X-Rays tend to be about their transmission - whereas for opal, I was trying to get from reflective to transmissive.

I couldn't see, eg, how the Bragg equation (or its derivations) could (& should) be used for the transmission case.

Maybe I should admit that I avoided reciprocal space.
(& hope to be able to continue to do so for this research, although I could be convinced otherwise, I guess)

Surely there are cases where X-Rays are reflected rather than transmitted by the diffraction, but I never did find out what those cases are.
(so that in itself would serve pretty well as the official question for this thread)