yeh, "a diamond's edge" is a very good translation of the lakorn title. also, in the thai language, putting a name with a popular saying like "a diamond's edge" doesn't really change the uniqueness of the saying. even though it translates funny in english, it's still effective because talking in third person is very common in thai.
and if you want to play with the title even more, you can say the title is "a diamond's cutting edge" since karat is referred to a type and size of a diamond's cut (like a real diamond). the nang'eks name is also karat so it plays with the title even more. hahaha