I have returned, and I come bearing .gifs!
Well, actually, .pngs, ha ha file format joke.

This was pretty much my best find tonight, a fairly good depiction of the in-game
Heroes emerald profile. It's more detailed than I remember, and it doesn't appear to be truly transparent, but highlights certain edges along its polygons to emulate a degree of transparency. The emerald shares roughly the same proportions as SA2's emeralds, as opposed to SA1's emeralds, which were relatively squat in terms of height.

...And this was my greatest disappointment. I wanted to capture a decent shot of the emeralds rotating around Sonic, Knux, and Tails in the cutscene before Metal Overlord in
Heroes, mainly so we could get a good look at them from the top. Unfortunately, this was the best I could do. The emeralds were probably standardized by this point, so I imagine that they're essentially the same as the SA2 emeralds in terms of cuts and general proportions, but there's no way to prove it looking at this.

For completion's sake, I also took a couple of screen captures of the "drained" emeralds of SA1. Here you can better see what I mean when I say that the SA1 emeralds were "squat".

And here's a slightly worse shot with the emeralds' outlines obscured by hands, the background colors, and each other. The only notable thing here is that the drained emeralds are clearly transparent, which I am fairly confident is not true of SA1's regular-schmegular textured Chaos Emeralds.

One more thing - I thought for the longest time that the "drained" emeralds in
Unleashed differed from those of SA1 in that they still retained their original colors, but were just really, really dulled. Looking back at the Unleashed opening cutscene, it doesn't appear as if this was really the case. Watching further along until we see the actual in-game emeralds (after Sonic crashes into Apotos) seems to confirm that the drained emeralds don't actually have color. If they do, the differences are too minor for me to notice.