Bricks, Masonry, Asphalt and Concrete - river rock that yellows

I have just finished rocking my fireplace with natural river rock. There is one rock that is normally grey with maroon streaks through it. After I acid-wash the rock, then rinse with lots of water, it dries to its natural color and stays that way for about a week. After a week, it's surface begins to progressively turn yellow. If I leave it, it just gets more yellow. If I re-wash it with acid/water, it goes grey again, then repeats the progressive yellowing process. I have tried a second rinse with baking soda and water, and a second wash with a mild soap solution. It always yellows again. FYI: It is not a sandstone.

What is causing this yellowing and how do I stop it?. I do not want to remove the rock.

Acid will sometimes give rocks and masonry a yellow tone.Try a product called sure-klean available at a quality mason supply yard.If that doesn't work you can always chisel the stone out carefully and replace with another