I wanted to add a note here--if you're having trouble finding palladium or a pink diamond, periodically look around the town map when you have the appropriate task.
After about three days of having no palladium, I was taking my sim over to the library when I saw a blue rock symbol labeled as palladium. So I trotted right on over an picked it up.
After a couple of days of no pink diamonds, the same thing happened to me. A blue circle on the map highlighted a pink diamond and I was set.
I don't know if the game will highlight the special seeds for lifefruit in this manner--I'm guessing that it won't, but I had it, so I didn't try to wait and see.