If they are the ones from Southern Exposure Seed Exchange called Amy's Sugar Gems, they are listed as OP and so should be a stable variety you can save seeds from. I found other sites offering them under that name as well.

I have not run across anything about a Sweet 100 background in any of the sites I checked. I suppose it's possible there might be another variety developed elsewhere under just the name Sugar Gem without the "Amy's" in front? Different tomato?

Yes, Amy's Sugar Gems. But that's good to hear that they are OP. Actually, my Southern Exposure 2015 catalog doesn't say OP about them, nor does it even label them as heritage. That's why I was asking.

On the online catalog website in the cherry section, it does say "Indeterminate, open-pollinated" right at the beginning of the description by the picture of Sugar Gem. So you're in luck!

Hmm. Amy's Apricot cherry sounds interesting too ....

Ah, great. Thanks! Now, Apricot Cherry is indeed marked as "heritage" in my printed catalog. Yes, that one seems to be a winner too.

But I'm still interested in anyone who's had some experience with, well, either.

I haven't grown Amy's Sugar Gem, yet, but I plan to in 2020. Maybe it'll be a good Mountain Princess or Nodak Early replacement. A friend gave me seeds that she saved, and loads of vendors sell it. So, I assume it's true-to-type. Tatiana's Tomato Base says it 's open-pollinated.