Violet glass is protective against UV penetration and deterioration of contents due to exposure to light. This makes for a very long shelf life for the precious essential oils. The bottles made in Europe to an exceptionally high standard and come with reducer caps that allow for the dispensing of single drops in the case of the 5 ml. bottles. The larger bottles come with pour caps for use in diffusers. Though the bottles are described as holding 5 milliters, they may actually hold 7-8 milliters of liquid.