McQuarrie didn't come up with that, which I'm not sure you meant, but the phrasing intimates. He was just depicting what George was describing to him. The cylinders on the back of the Stormtroopers' belts were lightsabers, per George. Before he decided to keep lightsabers exclusive to Jedi. But he still had the notion that maybe they were still in wider use elsewhere in galactic civilization, outside of our scope in the film. That made it into ADF's novelization, where Old Ben was explaining lightsabers to Luke. There had been a duelling culture, to settle minor disputes. Many of the nobility, such as Bail Organa, would carry their "laser swords" around with them, the way would have been done in the Renaissance here on Earth.
There was nothing mystical about lightsabers themselves -- just a Jedi's ability to do things with them beyond the ability of ordinary people. A bit like Zen Sword masters versus a Venetian noble.. but with a bit more IRL Wire-Fu.
The crystal thing started out as a misunderstanding of the source material by the good folks at West End Games. They screwed up several things when they were making their first edition of the Star Wars RPG, back in the mid-'80s. In trying to find any available information on how lightsabers worked, about all there was was the description in the Star Wars novelization, based on the early, more sword-like, concept of the lightsaber hilt. The description of what Luke was presented with was: a stubby handgrip containing the highest-rated power cell that size Luke had ever seen, surmounted by a disc "barely larger than his spread palm", one side polished ot mirror brightness, the other side dull and studded with jewel-like controls.
Somehow, the WEG folks interpreted "the reverse side" as meaning, somehow, the inside of the lightsaber. And "focusing jewels" suddenly existed. But these were the same guys who interpreted the caption on a picture of the Executor moving in formation with five Star Destroyers, one of the publicity shots from Empire, which said, "Larger and with more firepower than five stardestroyers [sic], Lord Vader's flagship, the Executor"... as meaning the Super Star Destroyer was five times longer than a regular Star Destroyer, and with five times as many guns. They didn't even think anything was amiss when, to make the SSD fit on the diagram of relative ship sizes in the Imperial Sourcebook, they had to drastically foreshorten it and remove an entire bank of engines.
And, because, as part of the Star Wars Renaissance, with all the new content coming out in the early '90s, Lucas Licensing instroduced a policy where any new book or comic or game had to jibe with what had come before (especially problematic, given how such a restriction hadn't been in place for the early proto-EU), so "jewel-focused lightsabers" and the "Five Mile Fallacy" of SSDs were locked into the EU. There was pushback against the latter, as later authors who knew they were bigger would use phrasing like "more than five miles long" and other such oblique language. But lightsabers were well and thoroughly screwed.
The Luke-finishing-his-new-lightsaber scene was a last-minute addition to ROTJ, put in during pick-up shooting at ILM and never in the script. I don't know nearly enough about what went into creating that scene and who all was involved, such as who designed the "opened" version of Luke's 'saber with the crystal visible.
I do not, inherently, have a problem with a jewel or crystal inside a lightsaber. Or even many. We here in Reality-Land use jewels all over the place in electronics, as timing control. Note those fine Swiss watches with "eighteen-jewel movement". Also amplification. Small energy goes in, big energy comes out. Crystalline lattices have a lot of free electrons. Metals are crystalline, too. Or vice versa. Chemistry is weird, and I love it. Like... Aluminum oxide is hard crystalline structure. With trace impurities of Titanium, you have sapphire. With Chromium, you have ruby. Different compositions, different properties. So crystals totally make sense in the "getting energy from the power cell to the emitter" portion of the circuit. I'm even fine with the Jedi using some rare and exotic crystal that "mundane" lightsabers don't, that has some greater connectedness with the Force, and thus giving the Jedi greater attunement with their weapons...
But no... Not optically-based, optically-focused, or the color anything to do with the crystal -- "bleeding" or attunement or otherwise. I can accept a certain amount of "the crystal chooses the Jedi", but not to the extent given in later stuff. A single retcon can be relatively workable. Two or three, if they agree with each other, can add richness and dimension. But with lightsabers, every addition, while in themselves not necessarily problematic, have all been derived from exactly the wrong standpoint, so they keep getting further and further off.
If a lightsaber "blade" is focused through the crystal, why does it self-terminate a meter or so from the hilt? There's nothing to stop it. If it interacts with physical objects, including other lightsaber blades, with destructive effect, it'd do the same to the crystal it's focused through. We don't need mystical hoodoo to make it work. Han was able to activate and use Luke's lightsaber, with no training. So you don't need to be a Jedi to do so. At least minimally. There were notions for a time, that I rolled my eyes at, that one needed to be a Jedi to even turn one on -- that a Jedi was channeling the Force through the crystal and manifesting it as a physical blade. Hah hah hah nope. I've said before, lightsabers exhibiting every property seen in the OT are already modeled through high-energy physics and technologies that are, admittedly, in their infancy, but the potential is easily calculable, and we're already seeing progress.
High-energy plasma blade "spinning" (as the arc is handed off to the next pair of emitter nodes that are all arrayed in a circle) at near-lightspeed, The faster the spin, the more blueshifted the spectrum of the blade. Base is yellow-green. Tuning it higher shifts into the blue. Higher still into the violet, but those would be scary unstable, due to the tremendous gyroscopic effect imparted. Jedi temple guardians have "detuned" lightsabers, to yellow, as a badge of their ceremonial duties and the fighting style that goes with the job. The exiled Jedi of the Hundred Year Darkness had to make do with scavenged equipment and materials and after a while their lightsabers were all so poorly tuned, they were red. After they had re-established their resources, they stuck with red as an identity statement.
I like the idea of kyber crystals being especially choice, due to how strong their amplification effect is. See also the Death Star and the Starkiller. I like the idea that the Jedi knew there were paraphysical properties to them, too, that enhanced their combat skills with the lightsabers. I like the idea that they're basically superconductors for the Force, such that Jedi meditating on it can find one that's especially in-tune with them, for optimum efficacy. But the more woo stuff they've been hitting us with more and more recently? So very no.