A row of paving slabs or paviours between the two areas would define them if you don't want height. Or use Victorian rope tiles, or the modern equivalent.

A picture would help. Perhaps something simple like garden kerb stones would allow the definition between lawn and gravel? It will certianly help stop the gravel and grass invading each other.

How about a couple of narrow borders between the lawn and gravel. You could have a path (pavers/brick etc - how about a large round stepping stone) between two borders which leads from lawn to gravel. If need be you could always edge the borders with some sort of border edging to help stop the stones getting into the grass. Otherwise something like reclaimed railway sleepers (presuming you want a straight line that is) dug into the ground. Sleepers are probably too big for your needs but you could probably find something similar, that is a more suitable size. Failing that how about a brick edge, the bricks can be dug in low enough to ensure there isn't a step but high enough to keep the stones contained, plus you can form them into a less formal line.