Where can I buy some edible "diamonds" like in the photo below? Or do you think they are real gems? Also, how do you attach them to the cake, maybe with some clear piping gel?

You can buy edible diamonds on some cake supply sites. I've purchased them from Kitchen Krafts before, and they worked really well. www.kitchenkrafts.com

Another option is to try and make them out of hard candy, but I'd definitely go the purchase route if it was me.

How do you attach them to the cake? Seems like you shouldn't use buttercream or RI since they are clear, it would show through.

I used a small amount of piping gel on my niece's wedding cake. Worked for me, they didn't fall off.

You can mix piping gel and disco dust, then dot it onto the cake and it appears to look like diamonds.

ETA: Ok, I think it's Lemon oil you mix not piping gel. Sorry.

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