Good and bad rocks

Unless you want the rocks to affect the water chemistry, the best test is to dump some vinegar on them. If you hear a fizzle,or see bubbles, it's not inert and will change the water chemistry.

Slate? - inert
Granite? - inert
Shale? - not inert
Fieldstone? - I don't think it's inert.
Flagstone? - will increase hardness
River rocks? - depends on the composition--river rock is usually granite.
Petrifiedwood? - inert
Pagoda? - no clue?
Lava? - may increase iron, and hardness. If it's 'real' lava rock, it will. They sell 'lava' rock that is a by-product of the smelting process, and it is normally inert.

As always, you can test any rock by determining the water parameters of a gallon of water, soaking the rock for a week, and retesting the water.
 
I often find that vinegar doesn't give enough sizzle for me to actually see it happening. I use stronger things like sulphuric or muriatic. Of course I work in a lab so it is easy to get those. But if you stick with Oriongirl's list you should be fine. Granit is generally plentiful and possibly free. I use it in my tetra tank.

Quartz is kind of pretty as well.
 
I haven't ever seen bubbles, even with crushed coral. However, if I listen closely, especially in a tub, the fizzle is fairly audible.
 
I just put 2 drops from bottle #1 onto a piece of fieldstone.

No fizz at all. I'm assuming that it would fizz right away?

I also have had 2 pieces soaking in a small (just big enough to hold them) container of water for 2 days. no change in PH. I also keep a cup of water sitting out for 2 days with nothing in it just as a comparison.

I have noticed that the PH of my tap water goes up about .6 after you let it sit out for a while. I would have expected it to go down.

I guess I have to brush up on my chemistry.
 
The water is probably loosing excess CO2 which causes acidity. So the pH will go up a bit. Do you know your kh and gh values? If they are high it might take awhile before the rocks made any sort of incraese. Of course if they are that high then adding them won't be an issue IMO.
 
kh=5
dh=5

The fact that I had no sizzle from the HCL seems like a good indicator though. right?

The reason for all this is that all the LFS that I've been to have had a pretty bad selection of rocks and it's hard to find a landscaping type place around here in the winter (what rocks they have are covered in snow). I was able to get about 8 pieces of 6 x 8 x 1 of fieldstone for $2. They look real natural so I was hoping they would work out.
 
I would say use them but keep testing your tank water for the next while. If there are weird changes it will probably be the rocks.
 
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