Metal headband base. We also have 3mm in the same color, 3mm in silver, and 5mm available. As shown in the image, there may be differences in the opening width. Also, there are some minor scratches here and there, but we will sell them as items that do not have any problems with use, so please understand. Rust-preventative spray has been applied, but we cannot guarantee that it will not discolor. We also have a small pack of 3 pieces, which is more expensive. (The set of 3 will be shipped by regular mail, so anonymous shipping is not available) Diameter: Approximately 12.5cm (adjustable) Material: Iron #SunyaAccessoriesParts