The glass can be colourless or coloured, clear or amber. There are different types of closure systems, e.g. Screw vials (closed with screw cap or dropper/pipette), Lip vials (closed with cork or plastic stopper), Crimp vials (closed with a rubber stopper and a metal cap). A vial can have a tubular shape or a bottle-like shape with a neck. The bottom is usually flat unlike test tubes which have usually a rounded bottom. Modern vials can be made of plastic (e.g. polypropylene). These can have other closure systems like a flip-top/snap cap.