There's actually 4 official types of fires.
A - Mostly organics as well as plastics.
B - flammable liquid and gases
C - live electrical equipment or wires/lines.
D - exotic metals.
A house fire can be a, b, c, and d type of fire. Fire depts don't generally use those "type" of fire identifications. It's a whatever is burning fire (i.e. structure fire, vehicle fire, brush fire, grease fire, etc). The A, B, C, D classes of fire are used by fire extinguisher/extinguishing agent manufacturers to distinguish types of fires their extinguishing agents/extinguishers can/should be used on.
As stated earlier most if not all fire departments use water to extinguish residential structure fires. Things like foam are used also but I'd say 99% of it is biodegradable now adays.
Having spent 15 years in the fire service I honestly wouldn't even consider using the rock, tank, or equipment. The combustible material inside moderns homes produce very very toxic smoke. Some of the by products of burning carpet and furnishings are hydrogen chloride and hydrogen cyanide. I bet if you tested the water of the tank after the fire was put out the water would be very acidic. It's the smoke and the by products of combustion in it that kill people and destroy things the fire was never near. Cleaning it up may make it look new but I'd be concerned about traces of combustion by-products lingering. I've had a few sets of turn out gear replaced during my time in the fire service for that very reason.
I'm happy you and your family are safe and I'm sorry for your loss. If there's anything I can do to help please don't hesitate to ask.