A raised bed is most productive and attractive as a bottomless frame set into a shallow trench. The sides can be almost any durable building material, including rock, brick, concrete and interlocking blocks. Watering troughs or claw-foot tubs can work, as long as they have the capacity and drainage.
But by far the most common material for raised beds is lumber. The major caveat, since raised beds are often used to grow edibles, is to steer clear of wood preserved with toxins. Avoid creosote-treated railroad ties; opt instead for naturally rot-resistant cedar or redwood. Whether using pressure-treated or naturally rot-resistant wood, put the bed together with galvanized or stainless screws or bolts.