I've never heard of a non-Platinum Jade, yet. Doesn't mean it doesn't exist, of course. Onyx and Rosewood are the models which have been available with different magnets over the years. The older "long body" models (Rosewood and Onyx) are alnico, because it was physically a very long magnet cylinder compared to the compact samarium-cobalt and platinum.
With the exception of Onyx, I believe all stones (Onyx was first, then Jade, then everything else) can be assumed to have platinum magnets. A top-plate serial # beginning with P (e.g. P022) always indicates the platinum magnet. I sometimes see old Onyx short bodies for sale (gold pins is the clue), where the seller clearly has no clue what they actually are but listed as "Onyx Platinum" when they're almost certainly not.