The wood is 3/4" x 3" Brazilian tigerwood prefinished. At the time when I mitered and framed the hearth, I didn't think about what I was doing. Now I need to figure out the gap between the floor and the vertical brick. Main focal point will be the front side, as I will have furniture close to each side of the FP. The floor is mostly parallel to the front edge of the hearth, with 1/2" gap on the left front and ~5/8" gap on the right front edge. 1/2" gap tapered to less down the sides to the wall.

My inclination is to cover the gap with quarter round or base shoe, painted white to match the rest of the room. I will need to fill a bit of the gap so I can attach the molding. I could dig up the miter frame and re-do. As it turned out, the miter frame is not very pleasing to me. But I'm afraid of damaging the surrounding pieces. Any other suggestions from the pros?? BTW, first time diy-fer, so please be gentle.

The appearance in the pictures don't look as bad as in real life.

Since you didn't undercut the hearth, the only thing I see will work and look half decent, is to use an endcap molding butted to the hearth.

What exactly is endcap molding? Is that something I can rip out of my tigerwood and attach in there, perhaps with darker boards to blend in between the floor and the brick? The edge of the board that's in the gap currently has neither a tongue or groove. How will I go about attaching it (glue and/or top nail)?

Maybe I can rip a 1/4" - 3/8" thin strip of my prefin surface of tigerwood that will cover the gap and then some to attach to the top of the floor. Is that the function of endcap molding? How can I rip that w/o losing fingers ?

An end cap has the profile of an L turned on its side.

If I were you, under the circumstances, and considering tigerwood trims are probably impossible to get, I'd make my own. Rip a floor board to 2" width; long enough pieces to go all around the hearth. 3 boards, two for the sides and one for the front. Now set your table saw blade at about a 15 to 20 degree angle. Run the boards through standing tall, not flat. When done, you'll have put a beveled front edge on each board, tapering from the full 3/4" thickness down to 1/8". Now sand out the saw marks. Get them all. apply about 3 or 4 coats of poly to the beveled area to match the top. Might be best to just sand off all the original finish to ensure that all the color and sheen is the same (uniform). Now, in effect, you have made your own 3/4" reducers. You can picture frame the hearth with those, sitting on top of the floor boards. Floorguy's idea of using square nosings (end caps) would basically be the same appearance. It would be easier if you could actually find moldings already made for the flooring.