I made sugar gems and put one on top of each cupcake last night. When I woke up this morning, they had melted all over the frosting! Can you not put sugar candies on buttercream?
Anyway, I had to scrape off the frosting, make more and re-ice.
Sugar is hygroscopic--this means that it attracts and absorbs water like a magnet.
You gems just did what comes naturally--drew out the moisture from the air and the icing and the cupcake and turned into sugar syrup.
Sugar pieces should be put on something at the very last minute.
Isomalt is less hygroscopic, so it can be put on earlier in the process, but they, too will go cloudy and over time, lose details.
Thank you so much, Rae! Now I'll know what to do and, more importantly, what NOT to do next time!