The edging material you choose for your pond should be in harmony with the pond itself. An informal shaped pond in a natural setting surrounded by square pavers results in a conflicting effect. Instead, select irregular cut flag stone, river rock 4 to 6 inches in diameter or larger irregular stones and/or boulders. In short, use materials that appear in nature itself. You could even have a natural landscaped border of grass, wildflowers or moisture-loving plants around your pond.
For formal pools choose man-made materials, for example symmetrical pavers, tiles, cut concrete blocks, etc.
No matter what materials you decide to use, the main objective is to make sure that the pond liner, pre-formed pond or concrete is not visible above the water line.
To accomplish this either blend the border into the edges of the pond or, when using pavers, overhang them over the edge of the pond.