Hi all,
is there a stone that won't raise the hardness any further?
Yes, basically it is all of them.
The exception is when you add CO2.
If you add CO2 the pH will fall, if you add enough CO2 to get below pH7, softer limestone will start to dissolve. Have a look at <"
this thread"> for a more complete explanation.
If you are worried, <"
rounded cobbles"> are always pretty safe, because only hard rocks form rounded cobbles.
You can tell that hard water doesn't dissolve limestone by the huge shell beds (below) that have built up over millennia in Lake Tanganyika.
Technical bit
Limestone (calcium carbonate - CaCO3) is only soluble in weak acids, it isn't soluble in pure water. Rain-water (pure H2O) is a weak acid, because as it falls a very small proportion of the dissolved CO2 becomes carbonic acid (H2CO3), and that then disassociates into H+ (proton) and HCO3- (bi-carbonate) ions. Acids are defined as "proton donors", and we've gained a proton.
When the rain falls in an area with limestone geology, the rain-water rapidly dissolves away a fragment of the rock and the carbonate ion (CO3--) neutralises two H+ ions to give two HCO3- ions. This process usually continues until the ground water is fully saturated with HCO3- ions and the water is "very hard" with a dKH of ~18. This is the situation for most the Southern and Eastern UK.
Because most of our limestones are fairly pure CaCO3, dKH and dGH will be the same (they are <"
both defined in terms of CaO">).
cheers Darrel