Take a look at some of the finished
projects over on Ravelry. There are some tips that may come in handy later.
If you look at some of the shawls, the center of the circle does seem to be a 7 times repeated motif.
http://www.ravelry.com/projects/anni...e/diatom-shawl
http://www.ravelry.com/projects/Idyl...g/diatom-shawl
http://www.ravelry.com/projects/Chilana/diatom-shawl