Well, if a magnet does not stick to them you can rule out aluminum. It's very likely that they are cast iron. To be sure, give the last 8 of your VIN to Rene,
@915_Tahoe .
If they are cast iron, the GM OE ball joints are 19209396 and the ACDelco Professional line is 45D2272.
The lower arms complete are 20869201 and 20869202. If your lower joints are bad, your uppers are very likely to be worn out too. These are not replaceable, you have to change out the whole arm. 25812725 and 25812726 are the GM OE numbers.
My research indicates that the GM OE arms last about 150-160K miles. The ACDelco Professional and Moog CK Series last about 80-100K miles. Some say less.