How can I anchor something in the gaps between the bricks or rocks of a fireplace without damaging the masonry?
Adding an answer because it would have been quite a long comment...

I knew I had seen "brick clips" before, but I'm not sure either style would work for you.

This kind is made to "hug" the top and bottom of a standard brick, so probably will not work for you, but at least you know it's available...



There's also this style that has little "ears" that you use to wedge it into the horzontal mortar lines.  These could work, but I'm not sure they will support the weight of tools, and may easily pull out.  I've only seen them used for Christmas decorations.



My goal here is to show you what I've seen, so maybe you can use them as search points for something that will work.