Jim, great suggestions about using metal! And of course stone, brick is lovely and inert. Limestone is a pain - I would avoid it at all cost (splinters and splits). Out here in the tundra, watch the use of cement and limestone for the high pH problems. Even planting next to a cement wall can kill a clematis around here...And another thought - I just learned a new-to-me trick for setting posts (wooden ones) in class 5 gravel rather than cement - dig the hole below the frost line, add a bit of clear rock (2-3 inches, set the post and fill around it with the class 5. This will allow the water to drain away from the post, greatly adding to the longevity. Perfectly stable if you tamp it correctly.
Best, Lisa in Mpls.