These are typically thought of for teenagers who want to show that they are serious about a relationship. But what about adults? Is the practice of buying a really dear friend a friendship ring too juvenile to do?
Well... hopefully people who are truly adults will do what they want without worrying too much about convention or what people (other than the giver and the recipient) think of their actions... at least as far as gifts of jewelry are concerned.
We occasionally get people here who are looking for something along the lines of what you're describing... sort of a pre-engagement ring. As I recall, the general advice is to get a ring that doesn't look too much like an engagement ring.
I also recall one instance when the "friendship ring" backfired, and apparently revealed that the intended just wasn't that into the giver...
With the popularity of Pandora and other cheaper / more accessible brands (in Australia) giving rings to friends is quite normal. I think the 'key' is to make sure they know it's because you value their friendship, and not as a 'if you wear this it means you love me' gift. The receiver should feel no obligation or pressure to wear it.
I've received a few rings over the years and have started a bit of a stack on my right hand - it's nice to be able to look down and be reminded of my journey and travels