These Crush-Proof Tubes are made from medical grade polypropylene (PP) and include an attached cap for secure storage. All tubes feature a frosted writing surface on the tube and are graduated. Tubes are black so as to keep the light out. All tubes are certified RNase-, DNase- and endotoxin-free for highest sample integrity. These tubes are often used for long-term storage of things such as DNA which gives you an idea as to their quality.