Most of us have seen these diamonds on trucks as we pass them on the highway. These diamonds are useful for safety and response teams to identify what is in the truck so the correct precautionary and clean up measures can be made.
The NFPA 704 fire diamond (or hazmat diamond) is described in NFPA Standard 704, maintained by the National Fire Protection Association. The system identifies four key hazards (health (blue), flammability (red), instability (yellow), and special (white)) and their degree of severity. Hazard severity is rated numerically, ranging from 0 (minimal) to 4 (severe).