The science-y part: Fire agate is a form of agate (which is formed of chalcedony and quartz, and grows in bands). This has a MOHS hardness between 5-7 which means it is less fragile and should scratch less easily. It's a cryptocrystalline silica formed in the cavities of volcanic rock. (Cryptocrystalline means that the crystals are so very small that their structure can only be seen in a very thin slice, under a microscope, and with a specific light.)
The magic part: Agates in general are grounding stones that can help keep you mentally and spiritually in the present moment. It stabilizes energy and is a good stone for connecting with who you truly are and understanding both your own truth but finding truth. It helps foster courage, and encourage inner growth.
Fire agates in particular have a deep connection to the earth. It has a calming energy that helps you feel safe and secure, and the grounding nature of agate is only enhanced by the strong earth energy. This is a gentle but powerful & passive protection stone. It acts to return curses, hexes and ill-thoughts back to the sender. Fire agate aids relaxation and helps with enhancing meditation sessions. It helps with spiritual fortitude, clearing blockages, and supporting you as you work through tough issues. Fire agate also has a connection to the element of, unsurprisingly, fire, and can be used for any spells of an intimate nature.