Old thread - ran across and adding comment for future... re soapstone:
We used to collect pumice stone in Oregon (another volcanic rock) which after floating for a while would often sink (absorb water in the air pockets).
Any rock from volcanic activity is general safe (ie inert due to high temps ie melted lava) but not so sure about soap stone (which I used to carve sculptures out of).
It is extremely soft and porous (so can leach out any one of the several heavy metals???) and is often different in composition depending on vein.
Here is info from quick search:
Note the difference in rocks that are of volcanic origin (melted, or with pumice - feather stone - liquid rock ie lava meeting cold water/air) verses info on non melted soapstone.
Soapstone (also known as steatite or soaprock) is a metamorphic rock, a talc-schist. It is largely composed of the mineral talc and is thus rich in magnesium.
It is produced by dynamothermal metamorphism and metasomatism, which occurs in the areas where tectonic plates are subducted, changing rocks by heat and pressure, with influx of fluids, but without melting.
It has been a medium for carving for thousands of years.
Soap stone is a tricky thing. I am a sculptor and have to be REALLY careful about where I acquire my stones as they might have any number of contaminates in them. With almost all stones I have carved there are deposits of heavy metals like iron or magnesium. This can leach out of the stone and into the water, so for this alone I wouldn't put any soap stones in my tank. You can never be sure by looking at the surface and soapstone can be porus.
Second and I am not sure of the reactivity of the chemical, but part of soapstones composition is a mineral called chlorite. This is what lends the nice greenish color to the stone. Most aquatic fish hate all forms of chlorine so if this component were to complex with any chemicals or biological agents in the tank it could produce some nasty chlorine by products that could wipe out the whole tank.
My opinion is a NO on the soapstone in the tank.
Soapstone is primarily composed of talc, which is a very soft, powdery-thready type of mineral. This is why it's so easy to carve, the same is true of gypsum. It will be prone to decomposition if left in water, ie it will literally come apart with time. The chemical makeup and crystal structure are related to asbestos, as is chlorite (no, neither of them pose a health risk to humans). This similarity means that it is very 'friable', that is as said above it is prone to disintegration. 'Soapstone' I believe contains many more minerals besides talc (which is why your sculpture is likely NOT disintegrating lol), but I definitely wouldn't put anything containing talc in my fish tank.