Seagrass is a plant that grows in shallow waters near coastlands. When it dries after being woven, it attains a brown, hard finish that is very similar in appearance to synthetic rattan, which can be made of Polyethylene or Polypropylene.
Some manufacturers, use the term Seagrass to describe just about any type of wicker furniture. Natural seagrass will have a mild hay-like odor even when it is dried and sealed with lacquer. Also, the grass reeds will have slight color variations.
Rattan, on the other hand, can be made to resemble seagrass but will be perfectly finished without any variation in color or texture.
Natural seagrass tends to fade when exposed directly to sunlight. Also, even the slightest exposure to moisture can cause rot to set in. Hence, these are best suited for indoor use or in a covered patio. If you are looking for a chair for outdoor use, go for synthetic rattan instead.