Rocks and boulders give a backyard pond a natural look while serving several useful functions. There are three kinds of natural rock. Sedimentary rocks like sandstone and limestone are the softest. Limestone should not be used in ponds with fish. Metamorphic rock -- like shale, slate, quartz and granite -- is harder and comes in interesting shapes. Igneous rock, like lava, is crystallised molten magma. Using rock that is native to your area creates harmony between the pond and surrounding landscape.
Use rocks to shape the pond, support it and blend it into the environment. Shelving the pond interior into gradual levels keeps the sides from collapsing and stacked granite field stone will hold the pond liner to the shelves. River rock and gravel on the bottom of the pond are a surface for the healthy bacteria that keeps the water balanced. Be sure the rocks are too big to be sucked into a submersible pump.
The pond perimeter gets the superstar stones. Boulders with colour, unusual shapes and texture draw the eye so don't place them near any flaws. Instead, use them for waterfalls, shelf overhangs and the centrepiece of a planted pond-side rock garden with flowering ground cover and ferns. Flat rocks make the weirs of a waterfall that sheet the water over the edge and down to the pond. Slabs of slate or granite pile nicely around the pond's perimeter to secure the pond liner, keep dirt and debris out, and provide a place to stand without eroding the edge.
Buried Boulders and Turtle Slabs
Rocks should look as artless as possible. Bury larger stones and boulders partway so they look as if they have been there for centuries. Scatter more stone outcroppings in the garden near the pond and plant them with ornamental grasses and wildflowers. Otherwise your stone water feature just pops out of the landscape like a lone mushroom. Use rounded field stone in the pond, and keep jagged riprap well away from the pond liner. Hang a bluestone slab over the water for turtles to sun on or hide beneath. Place a tall plinth stone in the pond, just under the surface, as a pedestal for a water plant.
Real rocks are heavy and expensive. Faux rocks are easier to install, come in a variety of pre-made and custom styles, and look like the real thing. They are basically reconstituted rock made of concrete, sandy rock material, fibreglass, silica and polymers. They are very durable, blend in with pond plantings and real rocks, and don't look fake. Some even come with the hose for a waterfall. Others are moulded to form the steps that support the sides of the pond. Contractors will customise your colours and finishes to match the stone in your area or the colour scheme of your pond.