Usually aluminum. It makes for minimal tungsten contamination of the weld and holds a balled tip. But the inverter based Tig welders don’t recommend balled tips for aluminum anymore - just normal sharpening with a slightly blunted nose. So if you’re still using the older transformer style machines they’re useful. But the rare earth ones are generally as good and work for both AC and DC welding.
(I’m in CT for the next couple of weeks.)