Si is the symbol for silicon, which is the second most abundant element in the Earth's crust (after oxygen.) The REEs are certainly not as abundant as that. Si is 270,000 ppm; cerium is 60 ppm.
Not a million times more rare, more like ~10,000 times. Also, Ni is more abundant in the oceanic crust than the continental crust. Lots of Ni in mantle rocks (peridotite is about 0.1% Ni.)
It's interesting comparing the abundance of copper (and even more so, tin) and iron. It was a huge transition in resource availability when the iron age began.
