I ordered a diamond impression mat but if it does not get here in time I am wondering how to do a diamond pattern on the cake without a mat.
If you have a picture of the pattern you are doing, I would think you could do it with your quilting tool, because, the mat is only going to put a light impression on the fondant as a guide. And then you would use this quilting tool to put the "actual line" deeper into it. So, if you have a steady hand, and "know" the pattern well, I say go for it!
I have used an argyle print scrapbook page in a pinch. 
Just go to the craft store and get a scrapbook page with a pattern that is the right size. Pin prick the pattern, lightly rub onto the cake and then trace with your quilting tool. Easy peasy...but not as easy as the impression mat. 
take a sheet of overhead transparency or any other similar acetate/plastic material (like the those plastic covers for term papers)
cut a square from it that is about 6 x 6 inches
now cut the square on the diagonal --
viola' -- a flexible template to mold to side of cake to guide the quilting tool that will give a 45 degree angle for the diamond pattern. with the right spacing they'll look like squares turned on point.
what's that you say - you want taller, narrower diamonds.
no problem -- the Pythagorean theorem to the rescue.
again, start with the same material as before....
cut so one side will be a full 8 inches long, the other a full 6 inches. then split on the diagonal. this gives a 10 inch long diagonal and the 30/60/90 combination to the interior angles.
put it against the cake so the 8 is upright and 6 is touching board for tall narrow diamonds.
lay it over so 6 is upright and 10 is touching the board for wide short diamonds
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final hint.
PREMEASURE just how far apart you want each line to start...this is where one of those cake divider mats comes in VERY handy.
the closer the lines, the smaller the diamonds (and oh the pain in the hand from doing all that tracing)
the farther apart the lines, the larger the diamonds.
you may want to take a cake pan or cake dummy and wrap it with a piece of paper and try out a few different spacings to get the diamond pattern you really like.
Wilton sells a set of square cookie cutters (or fondant cutters...I can't remember which). You can turn the size you need 1/2 a turn and voila' you have a diamond. Press it gently into the icing and it makes a diamond pattern. Thats how I made the diamond pattern on the princess cake in my photos.
If you are using it for BC, make sure to dust in powdered sugar first.
Good luck!!!
I can tell ya, he wasn't next to me in math class! Or I'd have gotten better grades! 
Thanks for the lesson Doug!
the 6/8/10 is an OLD trick based on the Pythagorean theorem -- used by carpenters everywhere.
it's the fastest way to check for / get something square when you can't measure the full diagonal in both directions.
just make a mark at some multiple of 3 on one side.
then a mark on the adjoining side at the same multiple of 4
then measure between the marks while adjusting -- when you get the same multiple of 5 -- it's square.
so if measure 3 / 4 -- should get 5
if 6 / 8 -- then 10
9 / 12 should be 15
and so on.....
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OH and you'd only want me next to you in Geometry!!!! -- the only math class I really understood! let's just say my Algebra, Trig and Calc teachers all just shook there heads in dismay.
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