ELI5 Rare Earth Metals - why they aren’t actually rare?

What makes these metals so special and why are they called ‘rare’? What technologies are they used in? Why are they so hard to substitute? Where are they found geographically? What about unobtanium and adamantium?  .. we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: wookiebadass, wrsaunders, commentingrobot, chauncy_prime, undyingcorn and diavolo_rosso_ To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]