I am especially interested about diamond gem trees, and do all gem trees spawn at about the same altitude?
I don’t think so. I believe they spawn at the same altitudes as normal sky islands, and are just as random. You’re just gonna have to get lucky
There appears to only be rarity. Depending on the type of gem tree, there can be more or less of a particular gem tree. Plus altitude doesn’t seem to much of an enforced rule though most might appear midway between ground and space. And I don’t think I have ever seen a diamond tree on the lower skyislands. Just mid or higher.
Though for gems, I would invest more in gem dodos. Lots of them. They help you get more shards to press into gems than you would get from waiting around a gem tree waiting for a gem to grow/spawn.