For reference, my own thoughts on the terms, and a general FAQ about gender free terms:
http://contradances.tumblr.com/post/113203981035/genderfree-contra-dance-term-faqI have not updated it with gems / rubies.
I like jets / rubies, but I think gems / rubies is better:
1. I disagree that the "em" sound is harder to hear than the "et" in jet. Good mic skills / having a foam pad on a mic will dull the sharp "ts" in "gents", and thus, "jets". Because a loud "ts" on the mic is harsh. Therefore, this argument against "gems" is not an issue.
2. A lot of people don't know "jet" is a gemstone, and so they think airplane. I've had a lot of gender free dancers complain about this. Given that the terms need to serve the LGBTQ community, and not merely us as callers, I take this complaint seriously. Thus, "gem" is a better choice.
3. Yes, a ruby is a gem. So what? They're both gems.
4. There's a gender connotation to thinking jet = airplane, since it's either phallic, or people think the NY/NJ football team, or the West Side Story fictional gang. Again, the terms are here to serve the dancers, not merely us.
5. Gem has all the same advantages as jet.
I thus think gem / ruby is a superior pair than jet / ruby.
Ron Blechner