"Earth" is an old word for ore. Rare earth elements are so named because their earths are rare: there aren't many places on the crust where their concentration

"Earth" is an old word for ore. Rare earth elements are so named because their earths are rare: there aren't many places on the crust where their concentration is significantly above the average. Mining depends on the existence of mineral earths. GP raises a valid point -- although a weak one.