There are actually different types of 'dragon stone' the one that is actually less expensive is not petrified wood, but rather a clay based stone. The later is a petrified wood. They look similar, but trained eye can see the difference.

Neither should impact your water table (mine never has and I have both)

 

thanks LBL, How can it be real if it doesn't have a wiki page? haha.

 

If you like it.... then its real. Unless you mail it off to a geologist it is difficult (please dont mail it off to a geologist). LOL.
There is a method to test for calcium, etc using muriatic acid (Hydrolonic available at most hardware stores... dont use vinegar). Just put on a drop and see if there is a reaction. In general, stone, wood, substrate release 'something'.... whether or not it is harmful to your tank is dependent on your water parameters and inhabitants.


I just got a 'onyxed petrified wood' that ended up screwing with my water column (which it should not have). Expensive mistake- but to their defense it was not sold as 'aquarium safe'.

Bump: Can you see the difference in this Photo? (in the same tank.... bought them both in Japan but from different sellers).
Also consider that different areas are the same stone but different compositions (like marble or granite)

 

You are in an area full of lots of differing ideas. I call it "marketing" more than solid facts. Kind of like the new idea passed around that there is no truth, just different versions? If I had a rock and it looks kinda/sorta like Ohko stone, I could sell it for a much better price if I called it "Ohko stone" even though it had never been to Ohko! So that leaves us very likely to find an imposter than the real item.
what that item is made of and how it may change your water if really an open subject with lots of answers as there are lots of rocks and lots of different water. So, it depends. If you get an actual hard, true, Ohko stone and put it in water with a fair amount of buffering quality, nothing happens but if you get an eroded limestone that looks like Ohko and put it in water that is soft and acidic, with little buffering, it may shoot the PH way up.
As you have found, there is very little consideration given to what the real stone actually requires to carry the name. It depends on the sellers ideas of truth, perhaps, and that seems to be an ever changing situation.

 

Clay based rock are primarily silicates and are not effected by acids formed in aquarium water.

Pour bit of white vinegar on rock, if it starts to fizz then it has potential to dissolve in aquarium and effect PH etc.

 

Now that it is way too late for me to retrain and do a different career, I find geology to be one that really catches my eye as it has so many different rocks to look at and study! And that rock that we often just drive past without even thinking has a lot to do with how we live and for sure changes what we do with our tanks.
One of the little secrets that I have discovered, due to living/working in a number of different parts of the US, is how much of our hobby is based on the rocks we find on both the East and West coast. Radical thinking, huh?
I look at it with the thought that much of our population and certainly lots of our media info are on each coast like New York. That makes much of that info written by folks whose experience is slanted toward what the water in New York does for them and that water is different because of the rocks that it sets in so much of the time! Granite and marble gives us different water than limestone!
So we get the info that we should test rocks with acid to see if they are good or not, even though it is a bogus test when done in large parts of the country where the water comes out of limestone! That is making a very general assumption and is certainly not uniformly true as the geology varies so much but it is true that a large portion of the center of the country sets on top of what was a large ocean bed at one time. That ocean left a really large amount of limestone and gives us a different situation. East coast folks may have to think about what it does to the water if they put a large chunk of wood in, but it matters very little if there is already lots of buffering in the water, like in the middle of the country. We just need to adapt to what we have, not what the books say!

 

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So we get the info that we should test rocks with acid to see if they are good or not, even though it is a bogus test when done in large parts of the country where the water comes out of limestone!

Its not that its a bogus test, its just not particularly conclusive in some cases, if you scratch your rock and put concentrated HCL on it and leave it and you see no physical change after an hour than its likely it will be inert in your tank as well no matter what kind of water you have.

However it would be better to put the rock submerged in the same pH water as your tank(after CO2 addition) and leave it there for a week to see if kH or pH changes.

It would be far better to use vinegar or ascorbic acid in your tap water(to get to your lowest pH) in a bucket and see after a week if the kH changed, the rock appearance changed, or the pH changed, that is a more indicative test.

 

Perhaps bogus is the wrong word and others might fit better. It only shows that pouring an acid on an alkaline does fizz but that is pretty low grade elementary school stuff that most of us should already know so it seems moving on to a more meaningful test would be better. Testing the rock in the normal water expected seems far more logical than testing in acid so I called it bogus! I felt bogus fit in somewhere between "not well thought out" and dumb!

 

If you want to be sure it WON'T react in your tank that high school chemistry will work. I'd probably chisel off a piece and crush it a bit and then put the pieces in concentrated HCL for an hour to be perfectly sure its inert, but that works no doubt.

The problem is you might get false positives, if you have hard water you can use many different rocks even Carbonate containing like Seiryu stone and everything could be fine. If you have soft water you have to be more selective or expect fluctuating kh which some people can balance okay but it takes experience.

Nothing wrong with the test its the problem with the interpretation and application to your water which trips people up.