I don't know what it is called but its the effect where you cross over lines and has an embossed diamond look sort of. anyway, what do you use to do thos lines???

You can either get an impression mat or use a ruler and make the criss-cross effect

It's called an embossing or impression mat I think - I just learned this the other day. It's a piece of clear plastic, about the size of a sheet of paper almost, with raised lines on it. You press it gently into either fondant or well-crusted buttercream. I am about to use mine that I just bought, we'll see how it goes!

You can find them at cake dec. stores, but I would call first and ask if they have one first. Also can be found online.