Here’s a finicky metal that won’t last long outside of the protective environment afforded behind a sealed vial. Praseodymium, like other metals in the rare earth family, is prone to oxidation. Break this ampule and within a day or two the metal pieces have turned black. A week later, they’ve turned to dusty rocks. A few weeks and there’s nothing left because the breeze blew it all away.

You can prevent total loss of the specimen by dunking it in mineral oil but oxidation still takes place, albeit at a slower pace. Still, the luster will be gone and the oxide will mix with the oil turning it into a cloudy mess unbecoming of a proper collection :-)