No safe is actually fireproof, as any material will burn if the temperature is hot enough. Fireproof gun safes are strong enough to resist the temperature of most home and office fires.
All fireproof gun safes receive a Underwriters Laboratories (UL) rating. The rating is based on the length of time and maximum temperature that a safe can withstand in a fire. The higher the rating, the stronger the protection.
Most inexpensive fireproof gun safes are insulated with either sheet rock or ceramic wool. Ceramic wool has a much higher temperature rating, but can cost more than a sheet rock insulated safe.
High-end fireproof gun safes tend to use a combination of concrete and vermiculite, both highly fire-resistant materials.
Regardless of the material, a fireproof gun safe can lose its effectiveness over time because of damage or age. Be sure to contact your manufacturer to verify how often you should replace your safe.