Ó 2005 kimlizzy (Kim Hollibone)

Open a new canvas and size to suit.

Insert shape/square

Remove the line

Resize to fit your canvas

Colour to suit and add filters if desired.

Go to Effects/Edges/Stamped

Now select the Erase Stamp

Select the diamond stamp

Click on "Customise the edge" and use the slider to adjust the size.

Done.