I found a Magma Crystal in the Volcano from beating the Lava Crystal building. I didn't get a picture because I went to check past Events.

I believe these are a retired event item, or if they are returning, shouldn't they be unable to collect until the event returns?

Umm I'm not sure but I think you can find them but they don't appear in your inventory until there's an event, or something like that. I might be completely wrong though, did you check your inventory, and if it is there, is there an option to use it? If not (as in you see it but can't use it), I think you can only use it during the event, but you can still find it anytime.

There are some in my inventory, I don't know how many I had before so I'm not sure if I actually gained one or not, but no option to use them