Fireplaces

Outside the clearance zone, yes. Within it, the material must be non combustible and set with a heat rated mortar. Follow the clearance figures the fireplace manufacturer gives, not a rule of thumb.

It is the traditional choice. Honed marble handles heat well and hides soot better than polished. Keep the stone sealed and wipe it down, since smoke residue is acidic over time.

Same principle: respect the clearance the unit specifies. Gas units usually run cooler at the surround, which widens the choice of tile, but the firebox itself still needs rated material.

A pH neutral stone cleaner and a soft brush. Skip acidic cleaners and abrasive pads, which etch and scratch. A sealed surround releases soot far more easily than an unsealed one.