The NT has the majority of rare earth mineral deposits in the world , , Funny you should mention this Peter , and the east coast of the north island of NZ has some as well , I used to work with an old rare earth prospector in a place named Te Teko ,
and yes it is used in glass manifacture , it allows FPL53 Ed to be as refractive as true Flourite but 4x stronger ,
eg its easier to figure and polish without the fragility and 50% + breakages that Flourite occures .
, I can imagine the curse's heard in a Japinese Vixen and TAK. optics lab before the advent of FPL51 and FPL53 .
Thanks for the rememberance there Pete .
Cool .
Brian.