Metal headband base. I also have 5mm in the same color, and 3mm and 5mm in gold listed. As shown in the image, there are variations 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. We also have a small pack of 3 pieces, which is more expensive. (The set of 3 will be shipped by regular mail) Diameter: Approximately 12.5cm (adjustable) Material: Iron #SunyaAccessoriesParts