Fire diamond is defined in NFPA 704. Proper labelling having the form of a diamond is used in our days in order to quickly identify the risks posed by hazardous substances.
This way, any emergency personnel (e.g. the fire brigade) that may be called in case of an accident can easily determine protective measures to be taken at the first critical moments of an emergency response.
NFPA diamond label is typically divided in four (4) areas, each one bearing a different colour.
The blue colour indicates the level of health hazard, the red one indicates flammability hazard, the yellow one the chemical reactivity hazard, whereas the white one contains special codes for very specific types of hazard. All four (4) hazards are rated on a scale from 0 (minimal hazard involved) to 4 (extreme hazard involved).
Typical NFPA diamond label (some of the abbreviations shown under Specific Hazard are not mentioned at NFPA 704 but are widely used nowadays)