Ooooo I would be very careful with bricks. You speak of caves, so I am assuming you mean those kinds of bricks with the little holes in them. Some kinds of bricks are very porous and will absorb anything they come in contact with. That's why you see some old houses with old red terra cotta brick that age & elements has caused to flake off big chunks of themselves like shale. The moisture in them freezing and thawing has caused them to crumble over the years.
So unless the bricks are brand new, you won't know what has been on them or soaked into them. Or what they might have been cleaned with, or treated/sealed with, or cemented with. You wouldn't want to have portland cement in your tank, as it would raise heck with your PH and hardness, if not be completley toxic. And even if the bricks are brand new, you don't know whats on them, or been splashed on them at the store/warehouse (like herbicides or other nasties from the garden department), nor what ingredients or residues might be left over from the manufacturing processes that would be harmful - like lead, which I think is still used in architectural cermaics.
HOWEVER....we aquarium nuts being the creative persistent obsessive types that we are

...you might be able to wash the brick(s) somehow. I would only do this with some new ones, however. Perhaps soak them in a bucket of fresh RO water. Or maybe boil them...not sure really. Or use vinegar or something natural like that. You obviously can't use soap and water. I'm not sure what would be appropriate and safe.
After they have been cleaned, soak them in a bucket of fresh water. Test the water for PH, GH, and KH before and after and see how they come out. Then if they test out ok, leave them in for a while and put in a couple quinea pigs like some goldfish to see how they fare. Then maybe a neon tetra or two - those would be a good test because they are so fussy. I wouldn't leave them in till they get sick, though, so I would keep an eye on them.
Oh, and here is another thought I just had. My father has a lake-front house on Lake Erie near Buffalo, and I grew up walking and playing on the beach. I can tell you there are always bricks washing up on the beach, or popping out of the sand after a storm, or being deposited by ice bergs after the winter. There are all different kinds of shapes and sizes, right up to complete cinder blocks. I would feel MUCH safer putting one of these bricks in my tank because they have been tossed, washed, ground, and scrubbed by sand, surf, and sun for who knows how many years in natural clean water. So if you live near any of the Great Lakes, or an ocean, go to the beach and look for some bricks there instead of buying them. Not only are they free, but you'll have a nice day at the beach too...
