Metal headband base. I also have 5mm and 3mm gold versions of the same color available. As shown in the image, there may be variations in the opening width. Also, there may be slight scratches in places, but we will sell them assuming they do not affect use, so please be aware of this. Sets of 10 of each are also available. Diameter: Approximately 12.5cm (adjustable) Material: Iron #SunyaAccessoriesParts