You can also use smaller pieces of flagstone, laid on edge like a a curb stone and buried over halfway into the ground, as an edging. But again, use big enough stones on the border, and no edging will be required. Follow the above link for more detail on this issue. Properly laid ...
Most of the edging you see in these photos was in place for the 12+ years we lived in our cottage. It held up well, was easy to mow and use a weed-eater around the lawn areas and worked great to keep the gravel paths and patio from seeping into the lawn....