I've always been curious - if triphosphor is also known as rare earth phosphor, how rare is it?

"Rare earth elements" is term used in chemistry for specific group of chemical elements.

Consequently phosphors using some of these elements got their name, hence "rare earth phosphors"

I guess it originate in the history, when the deposit they were found in was considered as rare.

How rare are they in fact today i don't know, but the fact, then the majority of them is mined in China would make them really rare for non-chineese industrial users...