I did the quest, and the Philosopher's Stone was in my family inventory, just as it was supposed to be. I'm assuming an old Sims hand like yourself looked there.
Hmm. The Philosopher's Stone is also a Lifetime Happiness reward, and it would be very easy to edit the file so that the cost of that reward was 1 point, while everything else was unchanged, if you really want the stone.
I'm finding it not all that interesting, though. It either makes ghosts members of your household or has a chance of turning a collectible item into a small bar of gold, and neither of those especially grabs me. YMMV, of course.