Position the glass fireplace doors inside the fireplace at the desired locations. Have an assistant hold the doors in place and test their operation by opening and closing each one to ensure proper clearance.
Reposition the glass doors and mark the locations of the screw attachment holes against the rock wall of the fireplace with a marker.
Drill into each marked area with a regular or hammer drill. Make sure that the drill is set to the highest power setting. Apply extra force against the butt of the drill and press it against the wall. Drill in spurts and back out the drill bit periodically to clean out the rock dust.
Monitor the condition of the end of the drill bit and replace it once it appears dull or if forward progress of the drilling slows to almost nothing. Multiple drill bits may be required to successfully complete the project, particularly if numerous holes need to be made.
Position the glass doors over their screw holes and set 3-inch masonry screws in place in the pre-drilled holes. Tighten them completely with the drill on its regular setting. Attach all screws partway until each one has been set in place then fully tighten them to complete the door installation.