I have an outdoor fireplace filled with shattered tempered glass. It's like, rough, chunky glass pebbles.

It's much much more complex that a solid sheet of tempered glass, and it catches the light and reflects it in sparkling ways.

Maybe these so called "complex" diamonds create more interesting light interfaces?

They don't.

If they did, it would be possible to detect the difference with perfect accuracy. Instead, the detectors made by those interested in pushing the concept of "real, natural" diamonds have a false positive rate of 5% looking for the inclusion of things labs could easily add if they cared to

If that's what you are after there are more interesting stones than diamonds for that, ie moissanite.