This is a 30mL cobalt blue PET boston round bottle and has a 13-415 neck finish. The boston round bottle is distinctive in its classic shape. Characteristic of the boston round are its rounded shoulders and rounded base. Polyethylene terephthalate (PET) is of the polyester family and has high clarity, good chemical resistance, high impact resistance, and medium rigidity. PET is also light-weight and is used for products containing essential oils. Avoid using PET containers to store products that contain essential oils greater than 5% or fragrance oils greater than 15%. If your oil percentages exceed these respective levels, please consider using glass. These bottles are sold by the case (100 count).