looks like the quatrafoil ( i know that is spelled wrong!) cutter set, pressed into the side of the cake. A big thing now. Really pretty!
There are others, more square than diamond, and fancy ones Search for fondant quilting markers.
Fancy Diamond Quilting Marker 3-in-a-row
APersonally I prefer to make my own template. The markers are awesome but never quite line up perfectly at least in one spot. At some point they have to overlap because of the circumference of the cake.
Once it's all coated, I measure the circumference. That way I know exactly how big the diamond need to be to match up.