Metal headband base. I also have 3mm and 5mm versions in the same color, as well as 3mm gold. As shown in the image, there may be slight differences in the opening width. Also, there may be some minor scratches here and there, but we will sell them as items that do not affect use, so please be aware of this. We also sell a small pack of 3 pieces. (The set of 3 will be shipped by regular mail, so anonymous shipping is not available) Diameter: Approximately 12.5cm (adjustable) Material: Iron #SnyaAccessoriesParts