How can I cover the gap between wood floor and round brick fireplace??

Perhaps this video will help Vera - https://www.youtube.com/watch?v=swhkyhxB8FY. You can also put some smooth river pebbles (the bigger ones) in the gap to hide it if you looking for a quick and easy solution

If you want to use wooden trim it's going to depend on how much of a curve you'll need. You'll want to find trim that is fairly thin. Then you will need to wet the trim, let it absorb the water then start to put an arc into the wood. Ideally you might make a jig to hold the trim. The "jig" could be nails hammered into an arc shape on another piece of wood. Start with a wide arc. Then let it dry, make a tighter arc shape, wet the wood again and see if you can form it to the newer shape. Keep doing this until it's the shape you want. FYI rockers for chairs are made using a form and steam. Wood can be bent but it maybe a slow process if it's a sharp curve.

Another way to bend wood is to cut several slots into the wood on the back side, that removes the "extra" and allows the back side or inside to contract. The slots need to be as close together as possible and wetting the wood, even with the slots, will help the fibers bend rather than break.

You'll need to toe in the nails to hold it in place or use glue but it's hard to get glue that will stick and old onto stone.

Last, there is vinyl trim available it may bend as much as you need without all the fuss that wood would take. Good luck.

Thank you so much for your suggestions. Will try one of them.

1/4 round? Maybe provide a pic