When I have needed to make diamond imprints, I took my square fondant cutters and squeezed them into a diamond shape. They bend back, and this has worked for me several times.
Good luck!
You can buy a diamond imoression mat form sugarshack at sharonssugarshack.com for like $5.00. The work well on fondant and buttercream and cover a large area all at aone time. I highly recommend them.
I have my first diamond pattern cake due the end of this month. I ordered the impression mat from cakes by sam and it was like $5 or $6 dollars. The only disadvantage I have found with ordering it online is the handling fees and shipping cost. So, this $5 dollar item is costing me like $15. But, when you gotta have it, you gotta have it.
I am looking foward to making this design, it's something different.
Happy shopping. 
Donnagardner,
When using these on fondant, do you roll the impression onto the fondant before putting it on the cake? If so, is the mat on the bottom or the fondant?
I guess with BC, you would push it up against the side of the cake after it's crusted, correct?
Thanks.
Well, I feel stupid! 
I made large diamond imprints on a cake for a customer who wanted an argyle-type pattern. That's when I used the my square fondant cutters.
I have the diamond impression mat from Sugar Shack and it works beautifully! I used it on buttercream and, yes, just let the icing crust and then use the mat. I put little pearls at the intersections. Everyone "oohed and aahed" and it was one of the easiest things ever!
Sorry I misunderstood the question, and, again, good luck!
Donnagardner,
When using these on fondant, do you roll the impression onto the fondant before putting it on the cake? If so, is the mat on the bottom or the fondant?
I guess with BC, you would push it up against the side of the cake after it's crusted, correct?
Thanks.
No, you do it right on the cake after it has been frosted or fondant applied. After your buttercream is crusted you can roll it along the side of the cake and push in slightly to put the impression on the cake, just be careful of the edges so you don't get a line from the edge. It can be used on fondant also but should be used fairly quickly after fondant is put on cake so it is still pliable. If the impression is not deep enough you can use a quilting wheel cutter thingy(?) to go over it to make the impression deeper. HTH Oh yeah...dust it with corn starch so it wont stick to the cake.
I just ordered these myself !!! 
http://www.pastrysampler.com/Images/5259s.jpg
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