I use galvanised steel lawn edging - freely available online and the likes of B&Q and Homebase. As flexible metal it can be used for curves very easily. It is also invisible but keeps the lawn from the border.
@Glasgowdan - my old garden had lots of stones that size (it was always a surprise in digging anything in my old garden - next door once dug up one boulder the size of an old fashioned TV, I once found the remains of a an unidentified animal). To make use of them I used them as border edging. Often the home of slugs and other undesirables. So the appeal of the above is very low for me!