Does anyone have a good way to adhere sugar gems to the sides of a cake that have been covered in fondant? I was thinking clear piping gel, but is it strong enough, and will it dry enough, to keep the gems from slipping off? Royal icing is strong, but it would probably show through the clear sugar gems, right?

maybe try some gum paste in water.

Use any sugar glue but be aware that moisture turns the gems cloudy so really you need to attach them as late as possible

Nuke 50/50 gumpaste to water, use glue sparingly, it's strong. This is what I used on my crown cake. Before serving brush gems lightly with light cooking oil, it will bring back the shine for a couple of hours.

Thanks so much for all the great advice!