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You could use other things such as brick, block or stone if you wanted too. However pressure treated lumber can be used, just keep any plantings at least a foot away from the wood, 2 feet if possible. PT wood contains arsenic that might leech into the soil and be sucked up into the plants, though that leeching drops off dramatically within a handful of inches from the wood. The arsenic doesn't "migrate" too far, but you don't want plant roots to grow out close to the wood either.