"Opals aren't hard enough," Milo answered, "If you wear them every day, they're going to get scratched and dented and destroyed, and if you want to wear the ring every day, like you're supposed to with... like... a wedding ring, then if you wear an opal you're going to end up having to replace it constantly. Diamonds are, I guess, the hardest possible stone, so that's why they're used. Well, that and propaganda," He looked at Xander, "but there are other stones that are almost as hard that work just as well, I think."
At Xander's question about missing anything, he fished around in his pocket for the museum map he had picked up once they were set loose, "No, it doesn't look like it. I guess machines are all we've left to see. That was fast..." then he checked his phone and realized exactly how many hours they had been there, "oh. Well. It felt fast."