I would use tylose mixed with water. You can make it as thick as you need it. Do a test run on a scrap of fondant before you actually need the cake, so you can see how long it takes to dry, if you have to hold the crystal on, etc.
I have used confectioner's glaze as glue for certain things and it glues super strong... now I don't know if it would leave a yellowish hue or not.... I would think since tylose glue might not hold it right away.... glue it with the tylose glue and use something to support it while it dries.... or angle it?
I have a batch of edible diamonds I bought and will be making a cake with them soon... curious to know what will work for you.... will keep an eye on this thread...
dalis4joe, what have you glued with confectioners glaze? I'm really interested to know because I use a lot of confectioners glaze (and have the headache to prove it!) but one of the features I like so much about it is that it's not sticky. Unless you get it on your fingers. 
Some of the round crystals will be used as center for small plunger flowers, so those won't be a problem hopefully b/c they can dry before applying to the cake. The others will be flat backed crystals (if I can find them) to be placed in a scroll pattern on the side of the cake.
I used conf. glaze mostly to seal the petal dust in my flowers because I don't steam my flowers... I thin it with conf. glaze thinner and use it like that to seal the petal dust... but I have used it full strength as glue for gold details on a cake and have also used it to make my cotton stamens I brush conf. glaze instead of egg whites or tylose glue... conf glaze is not sticky after it's dry... but it can be used as glue and it will hold.... my stamens that I make with cotton thread and dyed raw farina stay in place better with the glaze than with tylose glue or egg white... I read it somewhere and tried it and it's what I always do now....
I also use it full strength to make my fondant boards if I want them super shiny....
hth
I used conf. glaze mostly to seal the petal dust in my flowers because I don't steam my flowers... I thin it with conf. glaze thinner and use it like that to seal the petal dust... but I have used it full strength as glue for gold details on a cake and have also used it to make my cotton stamens I brush conf. glaze instead of egg whites or tylose glue... conf glaze is not sticky after it's dry... but it can be used as glue and it will hold.... my stamens that I make with cotton thread and dyed raw farina stay in place better with the glaze than with tylose glue or egg white... I read it somewhere and tried it and it's what I always do now....
I also use it full strength to make my fondant boards if I want them super shiny....
hth
Thanks so much for sharing your experiences.
I also use confectioners glaze for flowers and making pretty much anything shiny. I also use it with luster/highlighter dust when I want to seal the color and protect against smearing or fingerprints.
I'm anxious to try it as a glue now.
I tried tylose with water to attach a round crystal to a flower. It's only a 4mm crystal and a small plunger flower. It may work with a small flat bottomed crystal?
I'd like to see some pics of the cakes & flowers that the confectioners glaze was used on.
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