This medal must be kept in a sealed glass container or it would corrode to a white powder in a matter of days.
In fact, that's exactly what happened to my first europium coin. There was a hairline crack in the glass capsule (probably from the coin bouncing around inside it during shipping) and within a few days I could see that it was beginning to oxidize rapidly. After a week, so much oxide had built up that it had forced the two halves of the glass case apart like a clamshell, further hastening the reaction. Within a month, nothing was left of the coin but some pale yellow powder. So I saved it in a jar.