I make a lot of chess boards, and sometimes a really nice piece of wood will have a very small split or crack in the grain. I try to put those pieces on the bottom so that if I make a repair it won’t look as bad, and in most cases someone that doesn’t know there’s a repair there would never spot it.
The video here shows how make the repair using medium CA glue and an activator. Once the activator is applied you can see an immediate ‘drying’ of the glue, however deeper into the repair there is still some liquid glue, it takes a few seconds for the chemical reaction to reach the glue down in the crack. You can sand a repair of this kind in about 10 minutes of spraying the activator on.
This kind of repair looks good especially on darker wood that has grain texture, and will likely go unnoticed by anyone that doesn’t know a repair was made.